Setup Cognito Userpool「Only allow administrators to create users」settings with Amplify CLI

**Is your feature request related to a problem? Please describe.**

Some customers don't would like to allow users to sign themselves up because they only would like to issue accounts to users who have a contract with them. Amplify CLI does not appear to have the option to configure this.

**Describe the solution you'd like**

They hope to set up above settings without editing cloudformation which is created by `amplify add auth` command.

**Describe alternatives you've considered**

We would like to setup that settings like below,

```bash
$ amplify add auth

> Do you want to use the default authentication and security configuration? : Manua
l configuration

(some questions...)

> Do you want to allow users to sign themselves up?(Y/n) <-- New question.
```

**Additional context**
Add any other context or screenshots about the feature request here.