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