Allow auth triggers to be added individually

Currently when you do `amplify auth update` to add another auth trigger, you have to use the `Walkthrough all the auth configurations` option.
However the problem is if you have already configured some other triggers AND updated their configuration manually. In this case it is not clear how to avoid the "amplify auth update" not reverting some of those manual modifications.
It would be much better if instead there was an option to simply add a new trigger and configure it, without going through all the auth configuration options.

Start by reproducing `amplify auth update` and tracing the auth configuration flow for triggers. Add an option to configure one new trigger without walking through all auth settings, and verify that existing manually modified trigger configurations remain unchanged.

Written by the indexing model from the issue text.