Listener XDS metadata propagation

*Description*:

#3602 produced the design document [Metadata in XDS resources](https://gateway.envoyproxy.io/contributions/design/metadata/) 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](https://gateway.envoyproxy.io/docs/tasks/observability/proxy-accesslog/#additional-metadata).

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