Use BackendCluster to represent OIDCProvider

[zhaohuabing]

Using BackendCluster to represent OIDCProvider allows users to customize backend connection settings for OIDC providers, such as retry policy and tls setting.

Fix: #3178

[codecov]

Codecov Report

Attention: Patch coverage is 81.20567% with 53 lines in your changes missing coverage. Please review.

Project coverage is 65.65%. Comparing base (7293b6c) to head (977adb3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/xds/translator/oidc.go	69.36%	23 Missing and 11 partials ⚠️
internal/gatewayapi/securitypolicy.go	80.76%	10 Missing and 5 partials ⚠️
internal/gatewayapi/ext_service.go	91.66%	2 Missing and 1 partial ⚠️
internal/gatewayapi/envoyextensionpolicy.go	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4227      +/-   ##
==========================================
+ Coverage   65.56%   65.65%   +0.08%     
==========================================
  Files         200      200              
  Lines       30268    30362      +94     
==========================================
+ Hits        19845    19933      +88     
- Misses       9281     9285       +4     
- Partials     1142     1144       +2     

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[arkodg]

LGTM thanks !

[arkodg]

thanks @zhaohuabing !