Listener XDS metadata propagation

Description:

#3602 produced the design document Metadata in XDS resources which specifies how metadata from Gateway API resources should be translated to metadata on Envoy XDS resources. The metadata can then be used to enrich telemetry.

In the same pull request, the following propagations were implemented:

• HTTPRoute and GRPCRoute metadata --> Envoy Route metadata
• Gateway and Listener metadata --> Envoy Virtual Host metadata

I propose implementing propagation of the remaining ones as per the design document:

• Service, ServiceImport and Backend metadata + port name --> Envoy Cluster metadata
• Gateway (or GatewayClass for merged gateways) --> Envoy Listener metadata

Relevant Links:

https://envoyproxy.slack.com/archives/C03E6NHLESV/p1734608175712169