Support cross-namespace BackendRefs in HTTPRoutes

[kate-osborn]

Proposed changes

Allow HTTPRoutes to reference Backends in different namespaces if a ReferenceGrant permits it.

Problem: NKG does not allow HTTPRoutes to reference Backends in different namespaces.

Solution: Allow HTTPRoutes to reference Backends in different namespaces if a ReferenceGrant permits it. Both SecretObjectReferences and BackendObjectReferences are resolved through the referenceGrantResolver. The referenceGrantResolver transforms the map of ReferenceGrants into a structure that allows for O(n) lookup of references.

Testing: Added & modified unit tests. Made sure all ReferenceGrant conformance tests passed, including the new HTTPRouteInvalidReferenceGrant test.

Closes #695

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

[pleshakov]

Suggestion: since the PR name will end up in the changelog, perhaps "Support ReferenceGrants for allowing cross-namespace BackendRefs in HTTPRoutes" or simply " Support cross-namespace BackendRefs in HTTPRoutes" can better reflect the PR from the user perspective?