Use an `AWS::CodeStarConnections::Connection` for connecting an `AWS::Amplify::App` to an external repository

I'd like to be able to connect my Amplify Apps to GitHub via a CodeStarConnection resource, rather than passing a GitHub token secret through CloudFormation.

So when provisioning an Amplify App via CloudFormation, it could look like this:

```yml
AmplifyApp:
Type: AWS::Amplify::App
Properties:
...
ConnectionArn: arn:aws:codestar-connections:us-east-1:012012012012:connection/abc123
# AccessToken: abc123
# OauthToken: abc123
```

Or via the API

```sh
$ aws amplify create-app ... --connection-arn arn:aws:codestar-connections:us-east-1:012012012012:connection/abc123
```

Start by reviewing the AWS::Amplify::App CloudFormation resource and the aws amplify create-app command shown in the issue, focusing on their existing token-based connection properties. Done means both entry points accept a CodeStarConnections connection ARN and can provision an app without passing a GitHub token; confirm the current API and resource documentation before proceeding.

Written by the indexing model from the issue text.