api: cross-namespace policy target selectors

[guydc]

What type of PR is this?

What this PR does / why we need it:

• Adds the option to select targets across all namespaces
• Inspired by https://gateway-api.sigs.k8s.io/reference/spec/#gateway.networking.k8s.io/v1.RouteNamespaces
• In the future, we can extend this mechanism to support:
  • List of named namespaces
  • Label selector to select namespaces

Which issue(s) this PR fixes:

Relates to #5143

Release Notes: Yes/No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.84%. Comparing base (d50be42) to head (ba7b099).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5145      +/-   ##
==========================================
- Coverage   66.88%   66.84%   -0.05%     
==========================================
  Files         210      210              
  Lines       32998    32998              
==========================================
- Hits        22070    22056      -14     
- Misses       9589     9599      +10     
- Partials     1339     1343       +4     

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