Add an EntraID OIDC backend based on the included OpenIDConnectBackend

[ceko]

EntraID OIDC Backend

We have a need to use EntraID as an authentication backend, but EntraID does not closely follow the OpenID spec and can't be integrated with the current backends. In particular, the issuer used in discovery does not always match the issuer returned in other parts of the login flow.

I built a backend using the OpenIDConnectBackend as a model and integrated with the msal library. I also wrote several tests using the same backend's tests as a model for coverage.

Please let me know if this is something you'd like to include in the base product, if not I'll continue using it as a module in my SATOSA instance.

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