Add Init Step Customization

**Is your feature request related to a problem? Please describe.**
I am having an issue wherein there are certain restrictions on role creation in my enterprise, that requires the `Path` declaration to be part of the roles that are created for cognito upon rootStack instantion in the `amplify init` command. Due to the inability to customize much of anything about this command aside from region, and iam user **creating** the stack, this blocked my ability to init, and as such, I had to manually create the stack using the rootstack template in this repository, adding my `Path` to the auth and unauth roles, and then manually creating all the initial backend/.config files for amplify to think it had done an `init` itself.

**Describe the solution you'd like**
I'd like either the ability to generate an initial rootstack template, or the ability to provide one, **pre-init**. I also think alternately the necessity of these roles being created upon init, since they're purely necessary for cognito auth, may just need to be moved to being created when doing something like `amplify add auth` since its actually needed there. If I'm using something like **OpenID Connect** or a **Lambda Authorizer** these are just stale roles in my account.

**Describe alternatives you've considered**
The alternative I've considered and am in the process of vetting, is creating the stack manually myself, and manually using all the files `amplify init` creates with the necessary outputs to think its `init`'d itself.