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