Add Google OAuth example for additionalAuthorizationParams

[jhrozek]

Context

stacklok/toolhive#4862 adds additionalAuthorizationParams to OIDC and OAuth2 upstream provider configs. This enables passing custom query parameters in authorization requests — the primary use case being Google's access_type=offline for obtaining refresh tokens.

What needs documenting

Add a Google-specific example showing how to configure additionalAuthorizationParams for Google OAuth upstream providers. The example should cover:

• Setting access_type: "offline" and prompt: "consent" to ensure Google returns a refresh token
• Setting explicit scopes to avoid sending both the standard offline_access scope and Google's access_type=offline (they serve the same purpose via different mechanisms)
• A note explaining which reserved parameters are blocked (response_type, client_id, redirect_uri, scope, state, code_challenge, code_challenge_method, nonce)

Example CRD snippet:

upstreamProviders:
  - name: google
    type: oidc
    oidcConfig:
      issuerURL: https://accounts.google.com
      clientId: <google-client-id>
      scopes:
        - openid
        - email
        - profile
        - https://www.googleapis.com/auth/drive.file
      additionalAuthorizationParams:
        access_type: "offline"
        prompt: "consent"

Related

• Add AdditionalAuthorizationParams to upstream OAuth configs toolhive#4862 (implementation PR)
• Add additionalAuthorizationParams to upstream OAuth2/OIDC provider config toolhive#4840 (original issue)

[jhrozek]

Duplicate of #726. Being addressed in #731.