Support BackendTLSPolicy SubjectAltNames

[sjberman]

As an NGF user,
I want to specify Subject Alternative (SAN) names for my backends,
So that I can allow multiple alternative names to be present in the backend certificate.

Note: this may not actually be possible with nginx to support a list of SANs. See kubernetes-sigs/gateway-api#3591 which is changing this field to extended for this reason.

We still may want to investigate.

Acceptance

• Support the SubjectAltNames field in the BackendTLSPolicy

(Note: We may be able to only support a single subjectAltName, and this may be ok)
Note:

• See https://kubernetes.slack.com/archives/CR0H13KGA/p1755093585320019

[github-actions]

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