[firebase_app_check] Recaptcha Enterprise Support

[night91]

As documentation suggest, the dart package provides support for reCAPTCHA v3. I believe adding support for the enterprise version will lift the limitations of the free version. Firebase app check supports both reCAPTCHA v3 and reCAPTCHA Enterprise.

  /// Activates the Firebase App Check service.
  ///
  /// On web, provide the reCAPTCHA v3 Site Key which can be found in the
  /// Firebase Console. For more information, see
  /// [the Firebase Documentation](https://firebase.google.com/docs/app-check/web).