Fix attaching of HTTPRoutes to Gateway Listeners

[pleshakov]

Proposed changes

Problem:

• Rules for traffic attachment of HTTPRoutes to Gateways were clarified in Clarify Gateway Listener Status AttachedRoutes field kubernetes-sigs/gateway-api#2396 -- successful attachment should depend only on parentRefs in an HTTPRoute and AllowedRoutes of a Listener, even if either or both of them are invalid.
• The corresponding conformance test GatewayWithAttachedRoutes was added in Conformance: Adds Test to Exercise Gateway AttachedRoutes kubernetes-sigs/gateway-api#2477 , which fails for NGINX Gateway Fabric.
• NGINX Gateway Fabric will not try to attach an HTTPRoute to a Listener if either or both of them are invalid.

Solution:

• Make NGF compliant with the Gateway API and make the corresponding test pass.
• Introduce Attachable fields for Listener and HTTPRoute types of the Graph in the graph package.
• Update the validation logic:
  • NGF considers a Listener attachable if (a) its hostname is valid, (b) protocol is supported by NGF and (c) AllowedRoutes are valid.
  • NGF considers an HTTPRoute attachable if (d) its hostnames are valid.
• Attach an HTTPRoute to a Listener if both are attachable.

Note:
(a), (b) and (d) are not mentioned in
kubernetes-sigs/gateway-api#2396 However, they are necessary:

For (b), NGF doesn't know how to attach to non-supported protocols like TCP.

For (a), Listener hostname needed for HTTPRoute attaching, because it affects if an HTTPRoute can attach or not (per Gateway API spec). For (c), HTTPRoute hostnames are also needed, because they affect if an HTTPRoute can attach or not per Gateway API spec). See https://github.com/kubernetes-sigs/gateway-api/blob/52c2994ed9de1c287a37465490b91cfcf01bf16e/apis/v1/httproute_types.go#L71-L73

Testing:

• Unit tests are updated and extended
• Failing conformance test GatewayWithAttachedRoutes now passes.

CLOSES #1148

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[kate-osborn]

LGTM