Guide on how to setup custom message Cognito triggers

**Is your feature request related to a problem? Please describe.**
Currently there doesn't exist a guide that shows how to setup and send custom messages for Cognito triggers. Specifically for [`CustomEmailSender`](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-email-sender.html) and [`CustomSMSSender`](https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-sms-sender.html)

**Describe the solution you'd like**
A guide that walks through how to use Amplify CLI with `amplify override auth` to set the custom message triggers.

**Describe alternatives you've considered**
N/A

**Additional context**
From the `amplify-js` repo, there is an issue around this scenario where their use case is to use 3rd party providers to send messages, where the provided work around is a short explanation that would be useful as a more detailed guide.

https://github.com/aws-amplify/amplify-js/issues/3099#issuecomment-1386550265