feat(gateway): decoupled DNS hostname from listener hostname via annotation-only

[cleman95]

What does it do ?

When external-dns.kubernetes.io/gateway-hostname-source: annotation-only is set on a Route, ExternalDNS now creates DNS records from the annotation hostname (external-dns.kubernetes.io/hostname) using the parent Gateway's IP address, without requiring the annotation hostname to match the listener hostname.
Previously, matchRouteToListener required gwMatchingHost(listenerHostname, routeHostname) to return true before any endpoints were generated. When the annotation hostname differed from the listener hostname, the route was silently skipped, the gateway-hostname-source: annotation-only annotation was never effectively evaluated.

Motivation

Fixes #6594
The Ingress source already supports decoupling DNS hostname from routing hostname via external-dns.kubernetes.io/ingress-hostname-source: annotation-only, which creates a DNS record for any hostname in the annotation regardless of the Ingress spec host. The Gateway API source lacked this capability, making the following migration pattern impossible:

1. Listener routes traffic on mywebsite.example.com
2. ExternalDNS creates a DNS record for mywebsite.gtw.prod.example.com (in a zone managed by RFC2136)
3. An external CNAME (mywebsite.example.com → mywebsite.gtw.prod.example.com) is managed by a different DNS provider
   This pattern is common during Ingress → Gateway API migrations where the DNS zone managed by external-dns doesn't cover the routing hostname's parent zone, or where a CNAME chain exists between an externally-managed hostname and the RFC2136 zone.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

The existing docs/sources/gateway-api.md already documents the gateway-hostname-source annotation. The behavior change (bypassing listener hostname matching in annotation-only mode) could be noted there. Let me know if you'd like me to update it.

[kubernetes-prow]

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