caCertificateRefs ca.crt key can be customized

The current specification is very opinionated on the naming and doesn't allow to override the key name of "ca.crt" in secrets and config maps.

Se the [reference](https://gateway-api.sigs.k8s.io/reference/spec/#backendtlspolicyvalidation) "ConfigMap does not contain a key named ca.crt"

**What would you like to be added**:
An additional field with an custom name for `ca.crt`.

**Why this is needed**:
[OpenShift service serving certificate](https://docs.redhat.com/en/documentation/openshift_container_platform/4.20/html/security_and_compliance/configuring-certificates#add-service-serving) does generate `secrets` with the key `service-ca.crt`.

```
apiVersion: v1
data:
service-ca.crt: |
-----BEGIN CERTIFICATE-----
...
```

Start with the BackendTLSPolicy validation reference and the linked OpenShift service-serving certificate documentation, then review the issue discussion for the agreed API shape. Done means the specification clearly supports a custom certificate key while preserving the existing ca.crt behavior.

Written by the indexing model from the issue text.