feat: Reconcile BackendRefs on EnvoyProxy

[davidalger]

What this PR does / why we need it:

This adds support for BackendRefs on EnvoyProxies to the reconciler so the same endpoint discovery used for HTTPRoute refs will be used rather than DNS based discovery using service FQDNs.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 61.76471% with 91 lines in your changes missing coverage. Please review.

Project coverage is 66.65%. Comparing base (29946b0) to head (8491367). Report is 336 commits behind head on main.

:exclamation: Current head 8491367 differs from pull request most recent head e6040b2

Please upload reports for the commit e6040b2 to get more accurate results.

Files	Patch %	Lines
internal/provider/kubernetes/controller.go	25.86%	37 Missing and 6 partials :warning:
internal/provider/kubernetes/indexers.go	17.94%	30 Missing and 2 partials :warning:
internal/gatewayapi/listener.go	89.41%	7 Missing and 2 partials :warning:
internal/provider/kubernetes/predicates.go	72.00%	7 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3190      +/-   ##
==========================================
+ Coverage   66.51%   66.65%   +0.14%     
==========================================
  Files         161      162       +1     
  Lines       22673    22962     +289     
==========================================
+ Hits        15080    15306     +226     
- Misses       6720     6781      +61     
- Partials      873      875       +2     

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[davidalger]

/retest

[arkodg]

hey @davidalger @zirain, just realized that there's a massive overlap with this PR and https://github.com/envoyproxy/gateway/pull/3293 , is this anything from this PR that is still required ?

[github-actions[bot]]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!