Add OIDC token exchange methods for BYO-CIAM

FirebaseAuth/Sources/Swift/Auth/Auth.swift

@@ -2430,7 +2430,7 @@ extension Auth: AuthInterop {
   private var listenerHandles: NSMutableArray = []
 }
 
-/// Regionalized auth
+// MARK: Regionalized auth
 @available(iOS 13, tvOS 13, macOS 10.15, macCatalyst 13, watchOS 7, *)
 public extension Auth {
   /// Gets the Auth object for a `FirebaseApp` configured for a specific Regional Google Cloud