WIP: feat(appcheck-network): Extend NetworkClient for reCAPTCHA Enterprise

This commit modifies the `NetworkClient` to support fetching App Check
tokens using the reCAPTCHA Enterprise attestation type.

- Adds the `RECAPTCHA_ENTERPRISE_URL_TEMPLATE` constant for the new
  API endpoint.
- Includes `RECAPTCHA_ENTERPRISE` as a new `AttestationTokenType`.
- Updates the `getUrlTemplate` method to return the correct URL for
  reCAPTCHA Enterprise token exchange requests.

Author: hiteshmaurya56

appcheck/firebase-appcheck/src/main/java/com/google/firebase/appcheck/internal/NetworkClient.java

@@ -255,7 +255,7 @@ HttpURLConnection createHttpUrlConnection(URL url) throws IOException {
     return (HttpURLConnection) url.openConnection();
   }
 
-  private static final boolean isResponseSuccess(int responseCode) {
+  private static boolean isResponseSuccess(int responseCode) {
     return responseCode >= 200 && responseCode < 300;
   }
 }