CP/DP Split: optimize configuration events

[sjberman]

Problem: All config update events resulted in sending configuration to every Gateway, even if the change was irrelevant.

Solution: Compare new config with old config to determine if a ConfigApply is necessary. Simplified the change processor and handler to no longer have to determine this.

Testing: Verified that relevant Gateways are updated properly, otherwise no updates occur.

Closes #3272

Checklist

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• 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.

[codecov]

Codecov Report

Attention: Patch coverage is 86.82171% with 17 lines in your changes missing coverage. Please review.

Project coverage is 86.71%. Comparing base (6337c97) to head (f3b9b6a).
Report is 264 commits behind head on change/control-data-plane-split.

Files with missing lines	Patch %	Lines
internal/mode/static/nginx/agent/action.go	83.33%	8 Missing and 2 partials ⚠️
internal/mode/static/handler.go	87.09%	3 Missing and 1 partial ⚠️
internal/mode/static/nginx/agent/agent.go	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@                         Coverage Diff                         @@
##           change/control-data-plane-split    #3320      +/-   ##
===================================================================
- Coverage                            89.74%   86.71%   -3.03%     
===================================================================
  Files                                  109      129      +20     
  Lines                                11150    14751    +3601     
  Branches                                50       62      +12     
===================================================================
+ Hits                                 10007    12792    +2785     
- Misses                                1083     1816     +733     
- Partials                                60      143      +83     

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