feat: add body to ext auth

[AurelienPillevesse]

What type of PR is this?
Add gesture for body to ext auth

What this PR does / why we need it:

Which issue(s) this PR fixes:
Fixes #4431

[AurelienPillevesse]

The testing part (https://gateway.envoyproxy.io/contributions/develop/#testing) seems not to work on my computer so I tried to modify internal/gatewayapi/testdata/securitypolicy-with-extauth-backend.in.yaml and internal/gatewayapi/testdata/securitypolicy-with-extauth-backend.out.yaml manually and hope that it will pass.

Don't really know how to do it better.

[zirain]

it would be good to send API first.

[AurelienPillevesse]

From api/ or internal/gatewayapi/ or both ?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.27%. Comparing base (3074800) to head (f28348d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4671      +/-   ##
==========================================
+ Coverage   66.09%   66.27%   +0.18%     
==========================================
  Files         209      209              
  Lines       31890    31901      +11     
==========================================
+ Hits        21077    21142      +65     
+ Misses       9571     9510      -61     
- Partials     1242     1249       +7     

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[zhaohuabing]

cc @guydc

[guydc]

overall looks good to me, just one comment on the docs and please rebase + fix test code generation

[guydc]

LGTM, thanks.

[arkodg]

LGTM thanks !