feat(users): implement api route to add new user to cognito

[nganphan123]

Welcome to PrivacyPal! 👋

Fixes: #607

Description of the change:

Add new API route to add user to cognito under CLIENT group.

Motivation for the change:

We need this API for the client registration form. Professionals can add new clients given their username and email. It's not feasible to let professionals create usernames for clients, but Cognito doesn't allow changing this unless we create new user pool. Also, removing username from registration will requires changes in data schema as email now works as user identifier not username. I think we should keep the current process (letting professionals create usernames for clients) for the sake of upcoming peer testing.

How to test

PUT /api/clients

Request body required:

"username": new client username
"email": new client email

Request: http://localhost:3000/api/clients
{
    "username": "ngan-test-new-user",
    "email": "jill01009@gmail.com"
}

Response: 
{
    "data": {
        "message": "User successfully added to user pool"
    }
}

[connordoman]

Added new issue: #620

[connordoman]

Nice work Ngan! Thanks for getting docs done too