Enable OIDC provider support in the embedded auth server

[jhrozek]

Previously, the embedded auth server converted all upstream configs to OAuth2Config, which meant OIDCProviderImpl was never instantiated even when the upstream was configured as type "oidc". This caused OIDC features like ID token validation and automatic discovery to be silently skipped.

Refactor UpstreamConfig to carry an explicit provider type ("oidc" or "oauth2") with separate OIDCConfig and OAuth2Config fields. The upstream factory now dispatches on type, creating OIDCProviderImpl (with OIDC discovery and ID token validation) for OIDC upstreams and BaseOAuth2Provider for OAuth2 upstreams. Config validation enforces mutual exclusivity between the two config fields.

Tested end-to-end with Google as the OIDC upstream:

    upstreamProviders:
      - name: google
        type: oidc
        oidcConfig:
          issuerUrl: "https://accounts.google.com"
          clientId: "<google-client-id>"
          clientSecretRef:
            name: google-oauth-secret
            key: client-secret
          redirectUri: "https://example.com/oauth/callback"
          scopes:
            - openid
            - email

Fixes: https://github.com/stacklok/stacklok-epics/issues/235

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.30%. Comparing base (1e626bb) to head (12a5be9).
⚠️ Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/authserver/config.go	78.94%	2 Missing and 2 partials ⚠️
pkg/authserver/server_impl.go	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3720      +/-   ##
==========================================
+ Coverage   66.04%   66.30%   +0.25%     
==========================================
  Files         417      426       +9     
  Lines       41255    41765     +510     
==========================================
+ Hits        27247    27692     +445     
- Misses      11915    11958      +43     
- Partials     2093     2115      +22     

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