Add back the create subscription (#2993)

Related to flutter/flutter#132066

Will need this endpoint actively running in order to add additional logic

app_dart/bin/server.dart

@@ -7,6 +7,7 @@ import 'dart:math';
 
 import 'package:appengine/appengine.dart';
 import 'package:cocoon_service/cocoon_service.dart';
+import 'package:cocoon_service/src/request_handlers/github/branch_subscription.dart';
 import 'package:gcloud/db.dart';
 
 /// For local development, you might want to set this to true.
@@ -94,6 +95,11 @@ Future<void> main() async {
         githubChecksService: githubChecksService,
         scheduler: scheduler,
       ),
+      '/api/github/webhook-branch-subscription': GithubBranchWebhookSubscription(
+        config: config,
+        cache: cache,
+        branchService: branchService,
+      ),
 
       /// API to run authenticated graphql queries. It requires to pass the graphql query as the body
       /// of a POST request.