Support env variables in the whole app.config.yaml

(for now, this is supported only in runtime action manifest inputs)

As an example BringYourOwn AWS credentials are [read from app.config.yaml](https://developer.adobe.com/app-builder/docs/guides/configuration/#common-configuration), although we support the $include directive, which would provide a mechanism to users to not commit the secret into `app.config.yaml`, it would be good to support reading those from env.

This would align with our recommendation to store secrets in .env

Start at the app.config.yaml loader and the existing runtime action manifest input handling, where environment variables are currently supported. Trace how $include and credentials are read, then determine how environment values should apply to the rest of the configuration. Done means environment variables work throughout app.config.yaml without requiring committed secrets.

Written by the indexing model from the issue text.