Allow EPP to patch all listeners when target to GatewayClass

As a follow up of https://github.com/envoyproxy/gateway/issues/7900,
it would be good to allow patch all the listener for a gateway/gatewayclass with just one single EPP.

```yaml
apiVersion: gateway.envoyproxy.io/v1alpha1
kind: EnvoyPatchPolicy
metadata:
name: a-merge-gateways-epp
namespace: gateway-conformance-infra
spec:
jsonPatches:
# - name: "gateway-conformance-infra/teg-1/http-80"
- name: ""
operation:
op: add
path: /default_filter_chain/filters/0/typed_config/http_filters/0
value:
name: fake-filter-name
typed_config:
"@type": type.googleapis.com/fake-filter
...
type: type.googleapis.com/envoy.config.listener.v3.Listener
targetRef:
group: gateway.networking.k8s.io
kind: GatewayClass
name: a-merge-gateways
type: JSONPatch
```