Create User in Admin API

**Is your feature request related to a problem? Please describe.**
I have a few customer apps where new users have to be created in Cognito.
This is not a public app, allowing "sign-up" and Administrators of the app do not have AWS console access.

**Describe the solution you'd like**
Add create user to the Admin API quries.
Admin API already allows to restrict access to specific group ("admin" in this case) so Administrators can add users to groups... But they need the ability to create new users and set a temp password for the user to change when they first log in (same as workflow from creating a user in cognito)

**Describe alternatives you've considered**
Alternatives include
1: making direct queries to the cognito Admin API, but this requires additional steps outside the app bypassing the amplify api.
2: have users "register" themselves then have the admin grant access by verifying their registration and adding them to the correct group... This is the workflow we are trying to avoid.