Extend admin api to create new users

## Why you need it?
At the moment with the admin rest API https://github.com/apache/openserverless-admin-api you can only perform (from the readme):
- login
- change password
- build a custom image
- cleanup builds
- get info

This API could be extended to also perform user creation (and deletion) so that i can be integrated in other systems to automate the handling and deployment of new users.

## How it could be?
Implement the same functionality from the CLI ops admin adduser into the admin rest API service so it is performed behind a callable endpoint. I imagine 2 endpoints are enough:
- a POST to create users with a json body for the configuration of the services to deploy
- a DELETE to remove the user