feat(openapi): add OAuth 2.1 server endpoints and clarify OAuth modes (#2165)

## Summary
- Add OAuth server endpoints to openapi doc:
  - POST /oauth/clients/register - dynamic client registration
  - POST /oauth/token - token endpoint
  - GET /oauth/authorize - authorization endpoint
  - GET /oauth/authorizations/{id} - get authorization details
  - POST /oauth/authorizations/{id}/consent - approve/deny authorization
  - Admin endpoints: /admin/oauth/clients/* for client management

- Clarify OAuth documentation with two distinct modes:
  - oauth-client: Auth service as OAuth client (external providers)
  - oauth-server: Auth service as OAuth provider (for other apps)

- Add OAuthClientSchema for consistent client object structure
- Document configuration requirements (`GOTRUE_OAUTH_SERVER_ENABLED`,
`GOTRUE_OAUTH_SERVER_ALLOW_DYNAMIC_REGISTRATION`)

Author: cemalkilic

internal/api/oauthserver/handlers.go

@@ -216,6 +216,7 @@ func (s *Server) OAuthServerClientList(w http.ResponseWriter, r *http.Request) e
 	ctx := r.Context()
 	db := s.db.WithContext(ctx)
 
+	// TODO(cemal) :: Add pagination, check the `/admin/users` endpoint for reference
 	var clients []models.OAuthServerClient
 	if err := db.Q().Where("deleted_at is null").Order("created_at desc").All(&clients); err != nil {
 		return apierrors.NewInternalServerError("Error listing OAuth clients").WithInternalError(err)