Add endpoints for managing oauth users to VTN

[pohlm01]

This part is not part of OpenADR VTN itself, but there is no established protocol for the authentication server either. So for now we'll do a small build-in authentication server within the VTN.

Users should have:

• id (primary key, unique)
• Client ID (unique)
• Client Secret
• Reference (a name for the external system to recognize users by)
• Description (some additional context)
• Roles (list of roles associated with the user

In a future version we might want multiple credentials per user, possibly each with their own scope, but not for now