Imporve RBAC user REST endpoint to allow merging

At the moment the endpoint used to create and update users `/settings/rbac/users/{auth_domain}/{username}` does not allow merging so if a field is not provided its content its deleted.

This an issue in the CLI as to add roles, groups or simple changes in a user every single field must be provided this forces us to do a GET then replace the provided data and do a POST, this creates a race condition which is not ideal. We would like to be able to have an enpoint such as a PUT or PATCH where we could merge inividual fields with out affecting others.