Make healthz server port in file-provider configurable

When ran the script `make go.test.coverage` as is, got the same error as in the CI right now:

```
--- FAIL: TestNamespaceSelectorProvider (6.11s)
kubernetes_test.go:1338:
Error Trace: /home/eshaan/Documents/projects/gateway/internal/provider/kubernetes/kubernetes_test.go:1338
Error: Received unexpected error:
failed to create manager: error listening on :8081: listen tcp :8081: bind: address already in use
```

but when I modified the definition of the same to use only 1 process (via the `-p 1` argument)
```
.PHONY: go.test.coverage
go.test.coverage: go.test.cel ## Run go unit and integration tests in GitHub Actions
@$(LOG_TARGET)
KUBEBUILDER_ASSETS="$(shell $(tools/setup-envtest) use $(ENVTEST_K8S_VERSION) -p path)" \
go test ./... --tags=integration -race -coverprofile=coverage.xml -covermode=atomic -p 1
```

the same completes without any errors!

Logs

```
make[1]: Entering directory '/home/eshaan/Documents/projects/gateway'
===========> Running generate-gwapi-manifests ...
curl -sLo /home/eshaan/Documents/projects/gateway/bin/gatewayapi-crds.yaml https://github.com/kubernetes-sigs/gateway-api/releases/download/v1.2.1/experimental-install.yaml
mv /home/eshaan/Documents/projects/gateway/bin/gatewayapi-crds.yaml charts/gateway-helm/crds/gatewayapi-crds.yaml
===========> Running manifests ...
tools/bin/controller-gen crd:allowDangerousTypes=true paths="./api/..." output:crd:artifacts:config=charts/gateway-helm/crds/generated
===========> Running go.test.cel ...
Run CEL Validation on k8s 1.29.4
ok github.com/envoyproxy/gateway/test/cel-validation11.289s
Run CEL Validation on k8s 1.30.3
ok github.com/envoyproxy/gateway/test/cel-validation11.742s
Run CEL Validation on k8s 1.31.0
ok github.com/envoyproxy/gateway/test/cel-validation11.453s
Run CEL Validation on k8s 1.32.0
ok github.com/envoyproxy/gateway/test/cel-validation11.309s
===========> Running go.test.coverage ...
KUBEBUILDER_ASSETS="/home/eshaan/.local/share/kubebuilder-envtest/k8s/1.29.4-linux-amd64" \
go test ./... --tags=integration -race -coverprofile=coverage.xml -covermode=atomic -p 1
? github.com/envoyproxy/gateway [no test files]
? github.com/envoyproxy/gateway/api [no test files]
github.com/envoyproxy/gateway/api/v1alpha1coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/api/v1alpha1/validation1.074s coverage: 91.0% of statements
github.com/envoyproxy/gateway/cmd/egctl coverage: 0.0% of statements
github.com/envoyproxy/gateway/cmd/envoy-gatewaycoverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/admin 1.101scoverage: 57.1% of statements
ok github.com/envoyproxy/gateway/internal/cmd 3.121scoverage: 37.5% of statements
ok github.com/envoyproxy/gateway/internal/cmd/egctl21.959s coverage: 38.3% of statements
github.com/envoyproxy/gateway/internal/cmd/envoy coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/cmd/options coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/cmd/version coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/crypto 2.911scoverage: 67.0% of statements
github.com/envoyproxy/gateway/internal/envoygateway coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/envoygateway/config 1.124s coverage: 90.0% of statements
ok github.com/envoyproxy/gateway/internal/envoygateway/config/loader 4.147scoverage: 77.5% of statements
ok github.com/envoyproxy/gateway/internal/extension/registry 2.712s coverage: 14.0% of statements
? github.com/envoyproxy/gateway/internal/extension/types[no test files]
ok github.com/envoyproxy/gateway/internal/filewatcher1.033s coverage: 93.4% of statements
ok github.com/envoyproxy/gateway/internal/gatewayapi24.080s coverage: 91.2% of statements
github.com/envoyproxy/gateway/internal/gatewayapi/conformance coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/gatewayapi/resource 4.387s coverage: 9.2% of statements
ok github.com/envoyproxy/gateway/internal/gatewayapi/runner 2.485s coverage: 36.5% of statements
ok github.com/envoyproxy/gateway/internal/gatewayapi/status 1.130s coverage: 56.2% of statements
ok github.com/envoyproxy/gateway/internal/globalratelimit/runner 2.487s coverage: 40.5% of statements
github.com/envoyproxy/gateway/internal/infrastructure coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/infrastructure/common coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/infrastructure/host 1.903s coverage: 19.1% of statements
ok github.com/envoyproxy/gateway/internal/infrastructure/kubernetes 2.722scoverage: 65.5% of statements
ok github.com/envoyproxy/gateway/internal/infrastructure/kubernetes/proxy 2.898scoverage: 87.0% of statements
ok github.com/envoyproxy/gateway/internal/infrastructure/kubernetes/ratelimit 2.420s coverage: 87.4% of statements
ok github.com/envoyproxy/gateway/internal/infrastructure/kubernetes/resource 1.086s coverage: 93.5% of statements
github.com/envoyproxy/gateway/internal/infrastructure/runner coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/ir 1.126scoverage: 17.5% of statements
github.com/envoyproxy/gateway/internal/kubernetes coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/logging 1.082scoverage: 95.0% of statements
ok github.com/envoyproxy/gateway/internal/message 3.113scoverage: 45.6% of statements
ok github.com/envoyproxy/gateway/internal/metrics 1.163scoverage: 35.6% of statements
? github.com/envoyproxy/gateway/internal/provider [no test files]
ok github.com/envoyproxy/gateway/internal/provider/file35.825s coverage: 87.8% of statements
ok github.com/envoyproxy/gateway/internal/provider/kubernetes 35.421s coverage: 55.0% of statements
github.com/envoyproxy/gateway/internal/provider/kubernetes/test coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/provider/runner coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/troubleshoot coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/troubleshoot/collect coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/utils 1.085scoverage: 64.3% of statements
ok github.com/envoyproxy/gateway/internal/utils/env1.022s coverage: 100.0% of statements
ok github.com/envoyproxy/gateway/internal/utils/field1.023s coverage: 78.8% of statements
ok github.com/envoyproxy/gateway/internal/utils/file1.020s coverage: 78.6% of statements
ok github.com/envoyproxy/gateway/internal/utils/helm1.123s coverage: 10.4% of statements
ok github.com/envoyproxy/gateway/internal/utils/jsonpatch1.092s coverage: 87.7% of statements
ok github.com/envoyproxy/gateway/internal/utils/naming1.027s coverage: 100.0% of statements
github.com/envoyproxy/gateway/internal/utils/net coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/utils/path1.026s coverage: 73.1% of statements
github.com/envoyproxy/gateway/internal/utils/proto coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/utils/protocov coverage: 0.0% of statements
github.com/envoyproxy/gateway/internal/utils/ratelimit coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/utils/regex1.017s coverage: 100.0% of statements
ok github.com/envoyproxy/gateway/internal/utils/slice1.018s coverage: 100.0% of statements
ok github.com/envoyproxy/gateway/internal/wasm 28.293scoverage: 86.6% of statements
ok github.com/envoyproxy/gateway/internal/xds/bootstrap1.491s coverage: 84.1% of statements
github.com/envoyproxy/gateway/internal/xds/cache coverage: 0.0% of statements
? github.com/envoyproxy/gateway/internal/xds/extensions[no test files]
github.com/envoyproxy/gateway/internal/xds/filters coverage: 0.0% of statements
ok github.com/envoyproxy/gateway/internal/xds/server/runner 3.144s coverage: 12.3% of statements
ok github.com/envoyproxy/gateway/internal/xds/translator5.325s coverage: 83.9% of statements
ok github.com/envoyproxy/gateway/internal/xds/translator/runner 2.691s coverage: 69.2% of statements
ok github.com/envoyproxy/gateway/internal/xds/types1.150s coverage: 89.4% of statements
github.com/envoyproxy/gateway/internal/xds/utils coverage: 0.0% of statements
github.com/envoyproxy/gateway/proto/extensioncoverage: 0.0% of statements
github.com/envoyproxy/gateway/test/e2e/testscoverage: 0.0% of statements
github.com/envoyproxy/gateway/test/utils/kubernetes coverage: 0.0% of statements
github.com/envoyproxy/gateway/test/utils/prometheus coverage: 0.0% of statements
make[1]: Leaving directory '/home/eshaan/Documents/projects/gateway'
```

_Originally posted by @EshaanAgg in https://github.com/envoyproxy/gateway/pull/5126#discussion_r1929479266_

---

We need to make the healthz server port configurable, for now it is a static port 8081, may cause port conflicts problem when running k8s provider test in CI.