Allow use of SNS topics for AWS Amplify Build Notifications

**Is your feature request related to a problem? Please describe.**
The AWS Amplify console allows the submission of e-mail addresses to receive build notifications. After an address is entered, an SNS topic is created and the e-mail address is subscribed to it. It is not possible to specify an existing SNS topic.

**Describe the solution you'd like**
I would like to see the Amplify console allow the use of existing SNS topics for build notifications.

**Additional context**
None.

The issue names the AWS Amplify console and SNS topics but provides no repository files, tests, or entry points. Start by locating the build-notification configuration flow and determine how existing SNS topics are represented; done means the console can use an existing topic without creating a new one.

Written by the indexing model from the issue text.