feat: Add multiple providers and proxy improvements

[HowieHz]

• 支持 LINUX DO 论坛登录（支持 OIDC），此论坛流量情况。
• 支持 Google 登录（支持 OIDC）
• 重构代理相关，现在，以下请求也会走文档中提到的代理配置。
  • 用户信息请求
  • OIDC JWKS (JSON Web Key Set) 获取
  • OIDC Issuer 发现
• 相对应的修订文档
• 优先尝试 OIDC，失败降级为 OAuth2
• 实现 OIDC Issuer 发现
  • 配置了 jwkSetUri 时，直接从该 URI 获取 JWKS
  • 仅配置 issuerUri 时，自动访问 .well-known/openid-configuration 获取 JWKS 位置

使用户信息请求和 OIDC 请求能正确通过代理。
支持 LINUX DO 论坛登录和 Google 登录。
实现 OIDC Issuer 发现。

已经过测试：

[f2c-ci-robot]

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[HowieHz]

@JohnNiang

[HowieHz]

在线测试 -> https://howiehz.top/uc

[iLay1678]

pr规范要求必须附带release-note，可以参考下面的

自定义 SSO 支持配置 issuerUri、jwkSetUri 以及多个 scope

[Copilot]

Pull request overview

Adds new OAuth2/OIDC providers (Google, LINUX DO) and refactors OAuth2/OIDC network calls to consistently use the configured proxy, with corresponding documentation and UI asset updates.

Changes:

• Add Google and LINUX DO provider definitions (AuthProvider + Oauth2ClientRegistration) and icons.
• Route user-info requests and OIDC JWKS fetching through the proxy-configured WebClient.
• Update README to reflect OIDC support, new providers, and expanded proxy scope.

Reviewed changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/main/resources/static/linuxdo.svg	Adds LINUX DO icon for provider UI.
src/main/resources/static/google.svg	Adds Google icon for provider UI.
src/main/resources/extensions/client-registrations.yaml	Adds Google and LINUX DO client registration defaults.
src/main/resources/extensions/auth-provider.yaml	Adds Google and LINUX DO auth provider entries; fixes indentation for SSO config map.
src/main/java/run/halo/oauth/HaloOAuth2AuthenticationWebFilter.java	Uses proxy-enabled WebClient for token exchange + userinfo; introduces custom OIDC JWT decoder factory for JWKS retrieval.
README.md	Documents OIDC support, new providers, and expanded proxy coverage.

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