`Gateway.BackendTLS` is Support:Core without conformance tests

[candita]

What happened:

#3304 incorporated Support: Core API changes without any conformance testing.

• Gateway.BackendTLS is Core and experimental without a conformance test.
• BackendTLSPolicyValidation.SubjectAltNames is Core, but there is no conformance test.

It was somewhat discussed in https://github.com/kubernetes-sigs/gateway-api/pull/3304/files#r1739635258

Update: SubjectAltNames was changed to Extended in #3591

What you expected to happen:

We need to keep them both Gateway.BackendTLS as experimental Extended until there are conformance tests

Anything else we need to know?

BackendTLSPolicy is on track to go GA in v1.4.0 and this will block that.

[kl52752]

I'm working for test coverage for SAN field. If we could find someone to cover gateway.backendTLS field that would be great :)

[snorwin]

@candita @shaneutt @kl52752 Is SAN validation really Core? According to the API spec, the SubjectAltNames list is marked as Extended, whereas the fields within it are marked as Core. I think we should get a clear understanding of whether it is Core or Extended and update the spec accordingly.

	// SubjectAltNames contains one or more Subject Alternative Names.
	// When specified the certificate served from the backend MUST
	// have at least one Subject Alternate Name matching one of the specified SubjectAltNames.
	//
	// Support: Extended
	//
	// +optional
	// +listType=atomic
	// +kubebuilder:validation:MaxItems=5
	SubjectAltNames []SubjectAltName `json:"subjectAltNames,omitempty"`

https://github.com/kubernetes-sigs/gateway-api/blob/main/apis/v1alpha3/backendtlspolicy_types.go#L173

/cc @mlavacca