Support Gateway tls.backend field

[sjberman]

As an NGF user,
I want to specify client certificates for Gateway using the field tls.backend
So that I can easily present the gateway's identity to the backend pods.

The BackendTLS field supports mTLS when per-Service level BackendTLS Policy is set so that both Gateway and Backend pods can verify each others identity.

See the API spec

Acceptance

• Support the tls.backend.clientCertificateRef field of the Gateway resource
• ReferenceGrant allows ClientCertificateRef from another namespace to be attached, if specified

[github-actions]

This issue is stale because it has been open 14 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[salonichf5]

Putting this on hold due to discussions about change in spec field in gateway API team.

Will pick this back up once their change to the spec is merged