feat(sns): addSubscription returns the created Subscription (aws#16785)

Allow creating dependencies on the subscription by returning it from `addSubscription()`. Before this change the subscription resource was inaccessible. Creating a dependency required manually creating the subscription resource.

Our current workaround is:

```
        topic = sns.Topic(self, "topic")
        subscription_req = subs.SqsSubscription(queue)
        subscription_data = subscription_req.bind(topic)
        subscription = sns.Subscription(
            self,
            "Subscription",
            topic=topic,
            endpoint=subscription_data.endpoint,
            protocol=subscription_data.protocol,
            region=subscription_data.region,
            raw_message_delivery=subscription_data.raw_message_delivery,
            filter_policy=subscription_data.filter_policy,
            dead_letter_queue=subscription_data.dead_letter_queue,
        )

        something.add_dependency(subscription)
```

And it would be much simpler as:

```
topic = sns.Topic(self, "topic")
subscription = topic.add_subscription(subs.SqsSubscription(queue))
something.add_dependency(subscription)
```

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-sns/lib/topic-base.ts

@@ -31,7 +31,7 @@ export interface ITopic extends IResource, notifications.INotificationRuleTarget
   /**
    * Subscribe some endpoint to this topic
    */
-  addSubscription(subscription: ITopicSubscription): void;
+  addSubscription(subscription: ITopicSubscription): Subscription;
 
   /**
    * Adds a statement to the IAM resource policy associated with this topic.
@@ -68,7 +68,7 @@ export abstract class TopicBase extends Resource implements ITopic {
   /**
    * Subscribe some endpoint to this topic
    */
-  public addSubscription(subscription: ITopicSubscription) {
+  public addSubscription(subscription: ITopicSubscription): Subscription {
     const subscriptionConfig = subscription.bind(this);
 
     const scope = subscriptionConfig.subscriberScope || this;
@@ -83,7 +83,7 @@ export abstract class TopicBase extends Resource implements ITopic {
       throw new Error(`A subscription with id "${id}" already exists under the scope ${scope.node.path}`);
     }
 
-    new Subscription(scope, id, {
+    return new Subscription(scope, id, {
       topic: this,
       ...subscriptionConfig,
     });

packages/@aws-cdk/aws-sns/test/sns.test.ts

@@ -480,4 +480,26 @@ describe('Topic', () => {
 
 
   });
+
+  test('result of addSubscription() can be used as a dependency', () => {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const topic = new sns.Topic(stack, 'Topic');
+    const user = new iam.User(stack, 'User');
+
+    // WHEN
+    const subscription = topic.addSubscription({
+      bind: () => ({
+        protocol: sns.SubscriptionProtocol.HTTP,
+        endpoint: 'http://foo/bar',
+        subscriberId: 'my-subscription',
+      }),
+    });
+    user.node.addDependency(subscription);
+
+    // THEN
+    Template.fromStack(stack).hasResource('AWS::IAM::User', {
+      DependsOn: ['Topicmysubscription1E605DD7'],
+    });
+  });
 });