API for api key auth forwarding setting

[zhaohuabing]

This is required when

• the client identity needs to be used in a downstream filter or the backend service.
• we want to avoid exposing the API key to the backend service.

API for: #6461

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.11%. Comparing base (fc2766b) to head (90aeea0).
Report is 15 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6444      +/-   ##
==========================================
+ Coverage   70.78%   71.11%   +0.32%     
==========================================
  Files         220      220              
  Lines       37820    37892      +72     
==========================================
+ Hits        26772    26947     +175     
+ Misses       9488     9377     -111     
- Partials     1560     1568       +8     

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[zhaohuabing]

/retest