api: Support ForwardUsernameHeader field in the BasicAuth

[surenraju]

What type of PR is this?

Type of PR
Feature

What this PR does / why we need it:
#2947

This PR introduces the ForwardUsernameHeader field in the BasicAuth section of SecurityPolicy. It enables the Envoy to forward the username of a successfully authenticated user to the backend services via a specified HTTP header.
The field is optional. If it's not specified, the username will not be forwarded.

Which issue(s) this PR fixes:
Fixes 2947

User-Facing Changes
Added ForwardUsernameHeader in BasicAuth of SecurityPolicy to enable username forwarding to backend services.

apiVersion: gateway.envoyproxy.io/v1alpha1
kind: SecurityPolicy
metadata:
  name: backend-basic-auth
spec:
  targetRefs:
    - group: gateway.networking.k8s.io
      kind: HTTPRoute
      name: backend
  basicAuth:
    users:
      name: "basic-auth"
    forwardUsernameHeader: "x-username"

Release Notes: No

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.03%. Comparing base (12c4390) to head (0ba4d1d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5342      +/-   ##
==========================================
- Coverage   65.11%   65.03%   -0.09%     
==========================================
  Files         213      213              
  Lines       33586    33586              
==========================================
- Hits        21869    21842      -27     
- Misses      10392    10411      +19     
- Partials     1325     1333       +8     

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[zirain]

please run make docs to update the api doc.

[arkodg]

make generate should generate the remaining files here

[surenraju]

Thank you @arkodg @zirain. Updated the PR

[arkodg]

LGTM thanks !

[surenraju]

@arkodg @zirain thanks for the review. Are we merging this before or should I push other changes to same PR?