feat: add admission control and adaptive concurrency

[aburan28]

What type of PR is this?

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Release Notes: Yes/No

[codecov]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.98%. Comparing base (1d0f076) to head (d3ceb6c).
⚠️ Report is 141 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/gatewayapi/backendtrafficpolicy.go	93.10%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7275      +/-   ##
==========================================
+ Coverage   70.93%   70.98%   +0.05%     
==========================================
  Files         229      229              
  Lines       41044    41056      +12     
==========================================
+ Hits        29115    29145      +30     
+ Misses      10212    10198      -14     
+ Partials     1717     1713       -4     

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[github-actions]

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!