chore(sns): addSubscription does not return the subscription (#20819)

----





### Description
Related to the following issue: #20657. Added a return statement to addSubscription to return the Subscription

### All Submissions:

* [x] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*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

@@ -35,7 +35,7 @@ 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.
@@ -80,7 +80,7 @@ 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;
@@ -95,7 +95,7 @@ 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,
     });