API: Improve AppSync codegen step

**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 https://github.com/aws-amplify/docs/issues/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**
https://github.com/aws-amplify/docs/issues/2768