Check for ReferenceGrants from GRPCRoutes to Services

[kate-osborn]

Proposed changes

Problem: ReferenceGrants that permit GRPCRoutes to reference Services in different namespaces are ignored by NGF., effectively preventing all cross-namespace routing for GRPC traffic.

Solution: Check for ReferenceGrants from GRPCRoutes to Services when validating GRPCRoutes. If a ReferenceGrant exists that permits the cross-namespace reference, allow it.

Testing: Verified that a GRPCRoute that references a Service in a different namespace is allowed if and only if a ReferenceGrant exits for that particular reference.

Closes #2333

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

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

Honor ReferenceGrants that allow GRPCRoutes to reference Services in different namespaces

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.77%. Comparing base (9840a51) to head (3500d5e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2337      +/-   ##
==========================================
+ Coverage   87.74%   87.77%   +0.03%     
==========================================
  Files          96       96              
  Lines        6830     6847      +17     
  Branches       50       50              
==========================================
+ Hits         5993     6010      +17     
  Misses        780      780              
  Partials       57       57              

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[salonichf5]

lgtm