fix(parser) support same-name services across NSes

[rainest]

What this PR does / why we need it:

Correctly build upstreams for multiple backends where multiple services in the backends share the same name in different namespaces. Previously services were indexed by name alone in the rule builder, resulting in one clobbering the other(s).

Which issue this PR fixes:

Fix #3860

Special notes for your reviewer:

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Patch coverage: 100.0% and no project coverage change.

Comparison is base (7b111df) 66.6% compared to head (782c4d5) 66.7%.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4375   +/-   ##
=====================================
  Coverage   66.6%   66.7%           
=====================================
  Files        158     158           
  Lines      18563   18571    +8     
=====================================
+ Hits       12379   12396   +17     
+ Misses      5429    5426    -3     
+ Partials     755     749    -6     

Files Changed	Coverage Δ
internal/dataplane/parser/ingressrules.go	95.1% <100.0%> (+1.4%)	⬆️
internal/dataplane/parser/parser.go	82.0% <100.0%> (+2.8%)	⬆️

... and 7 files with indirect coverage changes

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[rainest]

This encountered a flake with TestGatewayWithGatewayClassReconciliation in envtests. Could not replicate, and don't think we have an equivalent of the integration dumps to try and analyze why it happens when it does.