WIP: feat(appcheck-recaptcha-enterprise): Add token exchange models and site key config

This commit introduces the necessary data models and configuration for
performing reCAPTCHA Enterprise token exchange with the Firebase App Check
backend.

- `SiteKey.java`: Provides a structured way to hold and manage the reCAPTCHA
  Enterprise site key.
- `ExchangeRecaptchaEnterpriseTokenRequest.java`: Defines the client-side
  model for the payload sent to the Firebase App Check Token Exchange API.
- `packageinfo.java`: Includes package-level metadata for the new
  reCAPTCHA Enterprise App Check module.

Further implementation, integration, and testing are still in progress.
This is not yet ready for review or final merge.

Author: hiteshmaurya56

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

@@ -0,0 +1,31 @@
+package com.google.firebase.appcheck.recaptchaenterprise.internal;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.VisibleForTesting;
+
+import org.json.JSONException;
+import org.json.JSONObject;
+
+/**
+ * Client-side model of the ExchangeRecaptchaEnterpriseTokenRequest payload from the Firebase App
+ * Check Token Exchange API.
+ */
+public class ExchangeRecaptchaEnterpriseTokenRequest {
+
+  @VisibleForTesting
+  static final String RECAPTCHA_ENTERPRISE_TOKEN_KEY = "recaptchaEnterpriseToken";
+
+  private final String recaptchaEnterpriseToken;
+
+  public ExchangeRecaptchaEnterpriseTokenRequest(@NonNull String recaptchaEnterpriseToken) {
+    this.recaptchaEnterpriseToken = recaptchaEnterpriseToken;
+  }
+
+  @NonNull
+  public String toJsonString() throws JSONException {
+    JSONObject jsonObject = new JSONObject();
+    jsonObject.put(RECAPTCHA_ENTERPRISE_TOKEN_KEY, recaptchaEnterpriseToken);
+
+    return jsonObject.toString();
+  }
+}

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

@@ -0,0 +1,17 @@
+package com.google.firebase.appcheck.recaptchaenterprise.internal;
+
+import androidx.annotation.NonNull;
+import java.util.Objects;
+
+public class SiteKey {
+  private final String value;
+
+  public SiteKey(@NonNull String value) {
+    this.value = Objects.requireNonNull(value, "Site key cannot be null");
+  }
+
+  @NonNull
+  public String value() {
+    return value;
+  }
+}

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

@@ -0,0 +1,2 @@
+/** @hide */
+package com.google.firebase.appcheck.recaptchaenterprise.internal;