feat(auth): add superadmin support for OAuth clients

* Introduce Superadmin field in OAuthClient configuration
* Implement IsSuperadmin method in OAuthRegistry
* Update identityAdmin to check superadmin status via OAuthRegistry
* Add tests for OAuthRegistry superadmin functionality

internal/config/config.go

@@ -66,6 +66,7 @@ type OAuthClient struct {
 	ID           string `yaml:"id"`
 	ClientID     string `yaml:"client_id"`
 	ClientSecret string `yaml:"client_secret"`
+	Superadmin   bool   `yaml:"superadmin"`
 	Description  string `yaml:"description"`
 }