WIP: feat(appcheck-recaptcha-enterprise): Implement RecaptchaEnterpriseAppCheckProvider

Implements the core `AppCheckProvider` for reCAPTCHA Enterprise,
orchestrating the attestation and token exchange process.

This class:
- Initializes the reCAPTCHA Android client using the provided `SiteKey`.
- Defines the `getToken()` method to:
    - Obtain a reCAPTCHA Enterprise attestation token from the Android client.
    - Exchange this attestation token with the Firebase App Check backend
      via `NetworkClient` using the `ExchangeRecaptchaEnterpriseTokenRequest` payload.
    - Convert the backend response into a `DefaultAppCheckToken`.
- Utilizes provided executors for asynchronous operations to ensure smooth
  integration with the Android application lifecycle.

Further unit and integration tests are still pending for this implementation.

Author: hiteshmaurya56

appcheck/firebase-appcheck-recaptchaenterprise/src/main/java/com/google/firebase/appcheck/recaptchaenterprise/internal/RecaptchaEnterpriseAppCheckProvider.java

@@ -0,0 +1,100 @@
+package com.google.firebase.appcheck.recaptchaenterprise.internal;
+
+import android.app.Application;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.VisibleForTesting;
+
+import com.google.android.gms.tasks.Task;
+import com.google.android.gms.tasks.Tasks;
+import com.google.android.recaptcha.Recaptcha;
+import com.google.android.recaptcha.RecaptchaAction;
+import com.google.android.recaptcha.RecaptchaTasksClient;
+import com.google.firebase.FirebaseApp;
+import com.google.firebase.annotations.concurrent.Blocking;
+import com.google.firebase.annotations.concurrent.Lightweight;
+import com.google.firebase.appcheck.AppCheckProvider;
+import com.google.firebase.appcheck.AppCheckToken;
+import com.google.firebase.appcheck.internal.DefaultAppCheckToken;
+import com.google.firebase.appcheck.internal.NetworkClient;
+import com.google.firebase.appcheck.internal.RetryManager;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Objects;
+import java.util.concurrent.Executor;
+
+public class RecaptchaEnterpriseAppCheckProvider implements AppCheckProvider {
+
+  private final RecaptchaAction recaptchaAction = RecaptchaAction.custom("fire_app_check");
+  private final Task<RecaptchaTasksClient> recaptchaTasksClientTask;
+  private final Executor liteExecutor;
+  private final Executor blockingExecutor;
+  private final RetryManager retryManager;
+  private final NetworkClient networkClient;
+
+  public RecaptchaEnterpriseAppCheckProvider(
+      @NonNull FirebaseApp firebaseApp,
+      @NonNull Application application,
+      @NonNull SiteKey siteKey,
+      @Lightweight Executor liteExecutor,
+      @Blocking Executor blockingExecutor) {
+    this.liteExecutor = liteExecutor;
+    this.blockingExecutor = blockingExecutor;
+    this.retryManager = new RetryManager();
+    this.networkClient = new NetworkClient(firebaseApp);
+    recaptchaTasksClientTask = Recaptcha.fetchTaskClient(application, siteKey.value());
+  }
+
+  @VisibleForTesting
+  RecaptchaEnterpriseAppCheckProvider(
+      @Lightweight Executor liteExecutor,
+      @Blocking Executor blockingExecutor,
+      @NonNull RetryManager retryManager,
+      @NonNull NetworkClient networkClient,
+      @NonNull RecaptchaTasksClient recaptchaTasksClient) {
+    this.liteExecutor = liteExecutor;
+    this.blockingExecutor = blockingExecutor;
+    this.retryManager = retryManager;
+    this.networkClient = networkClient;
+    this.recaptchaTasksClientTask = Tasks.forResult(recaptchaTasksClient);
+  }
+
+  @NonNull
+  @Override
+  public Task<AppCheckToken> getToken() {
+    return getRecaptchaEnterpriseAttestation()
+        .onSuccessTask(
+            liteExecutor,
+            recaptchaEnterpriseToken -> {
+              ExchangeRecaptchaEnterpriseTokenRequest request =
+                  new ExchangeRecaptchaEnterpriseTokenRequest(recaptchaEnterpriseToken);
+              return Tasks.call(
+                  blockingExecutor,
+                  () ->
+                      networkClient.exchangeAttestationForAppCheckToken(
+                          request.toJsonString().getBytes(StandardCharsets.UTF_8),
+                          NetworkClient.RECAPTCHA_ENTERPRISE,
+                          retryManager));
+            })
+        .onSuccessTask(
+            liteExecutor,
+            appCheckTokenResponse ->
+                Tasks.forResult(
+                    DefaultAppCheckToken.constructFromAppCheckTokenResponse(
+                        appCheckTokenResponse)));
+  }
+
+  @NonNull
+  private Task<String> getRecaptchaEnterpriseAttestation() {
+    return recaptchaTasksClientTask.continueWithTask(
+        blockingExecutor,
+        task -> {
+          if (task.isSuccessful()) {
+            RecaptchaTasksClient client = task.getResult();
+            return client.executeTask(recaptchaAction);
+          } else {
+            throw Objects.requireNonNull(task.getException());
+          }
+        });
+  }
+}