Support Gateway BackendTLS field

[sjberman]

As an NGF user,
I want to specify backend TLS settings at the Gateway level,
So that I can easily verify multiple backends by setting configuration in a single place.

The BackendTLS field gets overridden by a BackendTLSPolicy if one exists.

kubernetes-sigs/gateway-api#3563 has some information on this.

See the API spec

Acceptance

• Support the BackendTLS field of the Gateway resource
• ReferenceGrant allows ClientCertificateRef from another namespace to be attached, if specified

[github-actions]

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