Support configuring the server as an OpenID Connect Server #625
Description
Currently, the CasualOS server can only act as a OpenID Connect client (that is, a server which can accept OpenID Connect identity tokens and issue session keys in exchange). This gives us some flexibility in allowing CasualOS to utilize some other login system, but limits us if we want to go the other way.
This task is to investigate, design, and implement support for allowing CasualOS to act as a OpenID Connect server and issue access tokens which can then be used to gain access to various API functions.