[auth] support for oidc

[parisni]

Oidc is an auth method broadly used from large compagny to self hosted people.

Oidc adds value by delegating the auth to an external provider enabling single sign on, but also feature out of scope of IM such as double factor auth, brute force mitigation and much more.

While oidc is a key feature in other IM tools such as matrix on the XMPP side oidc is almost un-mentionned. There is AFAIK not any XMPP client to support OIDC right now. However there is some XMPP backend implementation such as prosody and also some manifests.

Sounds like oidc for mongooseIM makes lot of sense too, and hopefully this would motivate xmpp clients to implement it.