feat(extauth): add support for context extensions

[maxbrunet]

What type of PR is this?

Feature

What this PR does / why we need it:

Add support for ExtAuth context extensions in SecurityPolicy:

spec:
  extAuth:
    contextExtensions:
    - name: hello
      inline: world
    - name: foo
      type: ValueRef
      valueRef:
        kind: ConfigMap
        name: my-cm
        key: bar
    - name: bar
      type: ValueRef
      valueRef:
        kind: Secret
        name: my-secret
        key: foo

Which issue(s) this PR fixes:

Closes #6592

Release Notes: Yes

[codecov]

Codecov Report

❌ Patch coverage is 81.87500% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.36%. Comparing base (e5c1bb3) to head (0939490).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/provider/kubernetes/controller.go	60.97%	12 Missing and 4 partials ⚠️
internal/gatewayapi/securitypolicy.go	93.10%	2 Missing and 2 partials ⚠️
internal/provider/kubernetes/indexers.go	92.30%	2 Missing ⚠️
internal/xds/translator/credentialInjector.go	66.66%	0 Missing and 1 partial ⚠️
internal/xds/translator/custom_response.go	66.66%	0 Missing and 1 partial ⚠️
internal/xds/translator/extauth.go	93.33%	0 Missing and 1 partial ⚠️
internal/xds/translator/extproc.go	66.66%	0 Missing and 1 partial ⚠️
internal/xds/translator/lua.go	66.66%	0 Missing and 1 partial ⚠️
internal/xds/translator/oidc.go	66.66%	0 Missing and 1 partial ⚠️
internal/xds/translator/wasm.go	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7383      +/-   ##
==========================================
+ Coverage   72.30%   72.36%   +0.06%     
==========================================
  Files         231      231              
  Lines       33939    34071     +132     
==========================================
+ Hits        24539    24655     +116     
- Misses       7632     7640       +8     
- Partials     1768     1776       +8     

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