Add GEP-1713: ListenerSets - Standard Mechanism to Merge Multiple Gateways support

[DuncanDoyle]

kgateway version

1.19

Is your feature request related to a problem? Please describe.

GEP-1713: ListenerSets - Standard Mechanism to Merge Multiple Gateways introduces support for a concept called ListenerSets. This concept decouples the definition and configuration of Listeners from the Gateway CR.

This has a number of advantages:

• The current limit of Listeners is 64, which means only 64 SNI domains can be configured. We have customers that have many more domains on a single gateway.
• The Gloo Edge API allowed application teams to configure both the TLS Certs and SNI Domains as well as the routing logic in a single CR (VirtualService). With Gateway API, application teams can no longer own the configuration of Certs and SNI, as this needs to be configured in the Gateway CR (== platform team). ListenerSets would create the required decoupling to implement this separation of concern.

Describe the solution you'd like

Add support for ListenerSets as defined in GEP-1713: ListenerSets - Standard Mechanism to Merge Multiple Gateways](https://gateway-api.sigs.k8s.io/geps/gep-1713/).

Add the following support:

• Create an enhancement proposal that provides the API design and implementation.
• Add initial e2e tests for this feature.
• Update CI to run e2e tests.
• Add initial user docs.
• TBD

Describe alternatives you've considered

Do not support the Gateway API Inference Extension project.

Additional Context

• GEP-1713: ListenerSets - Standard Mechanism to Merge Multiple Gateways
• ListenerSet APIs + Generated Clients kubernetes-sigs/gateway-api#3588
• Support GEP-1713 Merge Multiple Gateways istio/istio#51016