Add option to request CAPTCHA on user creation

[RohanNagar]

Describe the problem that this feature would solve

One of the standard ways to prevent bot signups is to require the end user to answer a CAPTCHA sent from the server. Thunder does not currently have this functionality.

Describe the solution you'd like

There should be a configurable option to enable CAPTCHA on POST /users. Ideally, when enabled, the flow would look like:

1. Client application sends a POST request with the user data
2. The server responds with an appropriate error code asking the client to re-try with a solution to the given CAPTCHA challenge
3. The client displays the CAPTCHA challenge to the user, they answer
4. The client retries the request with the answer to the CAPTCHA

Additional context

I have never worked with CAPTCHA before, so I may not know all of the limitations or how the protocol should actually work.