Add support of Casdoor as provider

[gityzon]

Goals

To make the users of Casdoor get a more native experience of OIDC.

Non-Goals

No response

Background

Casdoor is an open-source UI-first Identity and Access Management (IAM).
It is excellent for use,it would be wonderful if Auth.js could support it.

Proposal

Reference:

https://casdoor.org/docs/how-to-connect/oidc-client