Add method to call the GeneratePlayIntegrityChallenge endpoint to NetworkClient.

Author: rosalyntan

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

@@ -57,6 +57,8 @@ public class NetworkClient {
       "https://firebaseappcheck.googleapis.com/v1beta/projects/%s/apps/%s:exchangeDebugToken?key=%s";
   private static final String PLAY_INTEGRITY_EXCHANGE_URL_TEMPLATE =
       "https://firebaseappcheck.googleapis.com/v1/projects/%s/apps/%s:exchangePlayIntegrityToken?key=%s";
+  private static final String PLAY_INTEGRITY_CHALLENGE_URL_TEMPLATE =
+      "https://firebaseappcheck.googleapis.com/v1/projects/%s/apps/%s:generatePlayIntegrityChallenge?key=%s";
   private static final String CONTENT_TYPE = "Content-Type";
   private static final String APPLICATION_JSON = "application/json";
   private static final String UTF_8 = "UTF-8";
@@ -119,6 +121,29 @@ public AppCheckTokenResponse exchangeAttestationForAppCheckToken(
       throw new FirebaseException("Too many attempts.");
     }
     URL url = new URL(String.format(getUrlTemplate(tokenType), projectId, appId, apiKey));
+    String response = makeNetworkRequest(url, requestBytes, retryManager);
+    return AppCheckTokenResponse.fromJsonString(response);
+  }
+
+  /**
+   * Calls the App Check backend using {@link HttpURLConnection} in order to generate a challenge
+   * nonce for the Play Integrity attestation flow.
+   */
+  @NonNull
+  public String generatePlayIntegrityChallenge(
+      @NonNull byte[] requestBytes, @NonNull RetryManager retryManager)
+      throws FirebaseException, IOException, JSONException {
+    if (!retryManager.canRetry()) {
+      throw new FirebaseException("Too many attempts.");
+    }
+    URL url =
+        new URL(String.format(PLAY_INTEGRITY_CHALLENGE_URL_TEMPLATE, projectId, appId, apiKey));
+    return makeNetworkRequest(url, requestBytes, retryManager);
+  }
+
+  private String makeNetworkRequest(
+      URL url, @NonNull byte[] requestBytes, @NonNull RetryManager retryManager)
+      throws FirebaseException, IOException, JSONException {
     HttpURLConnection urlConnection = createHttpUrlConnection(url);
 
     try {
@@ -163,7 +188,7 @@ public AppCheckTokenResponse exchangeAttestationForAppCheckToken(
                 + httpErrorResponse.getErrorMessage());
       }
       retryManager.resetBackoffOnSuccess();
-      return AppCheckTokenResponse.fromJsonString(responseBody);
+      return responseBody;
     } finally {
       urlConnection.disconnect();
     }