Create cognito user pool with AllowAdminCreateUserOnly=true

**Is your feature request related to a problem? Please describe.**
For partner or intranet dashboards/apps, I want to disable user sign-up and only allow admins to create users. User will then be added via an alternate way to the pool (AWS-CLI, Cognito console, alternate app).
`amplify add auth` doesn't allow me to do that

Is there a way to do this with custom CF templates? I didn't find out how to do this, if there is a solution, can you point me to it?

**Describe the solution you'd like**
When asked about the type of pool, I would like to see another option:
_Admin_ sign-up, sign-in, connected with AWS IAM ..._

It could also be a standalone question, but only when Federation won't be setup. I don't think you can have federation and admin-sign-up?

**Describe alternatives you've considered**
* Change the pool via Console: I need to remember to do this on every new provisionning
* Add some custom CF in my amplify code base. As said above, I couldn't figure out how to do that.

**Additional context**
This could perhaps have been a comment on RFC #766, but the thread is so big already and seems to be more dealing with administrative tasks than Cloudformation setup.