API: Improve AppSync codegen step

[diegocstn]

Is your feature request related to a problem? Please describe.
During the amplify push flow, the prompt "Do you want to generate code for your newly created GraphQL API" and the generated API file seem to confuse users when the project adopts DataStore aws-amplify/docs#2768 as amplify codegen models also generates the actual Swift models.

Describe the solution you'd like
We should skip the AppSync flows and not generate an API.swift unless the CLI detects that the project is using AppSync, or unless the customer explicitly opts in.

Describe alternatives you've considered
Improved documentation

Additional context
aws-amplify/docs#2768