TF-1289: [Data] Implement GetSubscription in fcm datasource

lib/features/push_notification/data/datasource/fcm_datasource.dart

@@ -17,4 +17,6 @@ abstract class FCMDatasource {
   Future<FirebaseSubscription> getFirebaseSubscriptionByDeviceId(String deviceId);
 
   Future<FirebaseSubscription> registerNewToken(RegisterNewTokenRequest newTokenRequest);
+
+  Future<FCMSubscriptionCache> geSubscription();
 }

lib/features/push_notification/data/datasource_impl/cache_fcm_datasource_impl.dart

@@ -59,4 +59,13 @@ class CacheFCMDatasourceImpl extends FCMDatasource {
        _exceptionThrower.throwException(error);
     });
   }
+  
+  @override
+  Future<FCMSubscriptionCache> geSubscription() {
+    return Future.sync(() async {
+      return await _firebaseCacheManager.getSubscription();
+    }).catchError((error) {
+      _exceptionThrower.throwException(error);
+    });
+  }
 }

lib/features/push_notification/data/datasource_impl/fcm_datasource_impl.dart

@@ -53,4 +53,9 @@ class FcmDatasourceImpl extends FCMDatasource {
   Future<bool> storeSubscription(FCMSubscriptionCache fcmSubscriptionCache) {
     throw UnimplementedError();
   }
+  
+  @override
+  Future<FCMSubscriptionCache> geSubscription() {
+    throw UnimplementedError();
+  }
 }