Support cross-namespace routing with TLSRoutes

[kate-osborn]

Proposed changes

Problem: With TLSRoutes, it's not possible to route to a backend in a different namespace than the route itself.

Solution: Allow users to grant TLSRoutes permission to reference backends in different namespaces using ReferenceGrants.

Testing: Ran conformance test and manual testing

Closes #2139

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

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please add a brief note that summarizes the change.

Support cross-namespace routing with TLSRoutes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.86%. Comparing base (4ac4b5f) to head (3792ed1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2379   +/-   ##
=======================================
  Coverage   88.85%   88.86%           
=======================================
  Files         100      100           
  Lines        7513     7515    +2     
  Branches       50       50           
=======================================
+ Hits         6676     6678    +2     
  Misses        781      781           
  Partials       56       56           

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