Add support for extracting authentication params from www-authenticate header

[romaindup]

When receiving a request with Authorization: Bearer, return an http 401 error with a header containing the necessary information to authenticate (WWW-Authenticate: Bearer, authorization_uri="https://...", client_id="...").
This allows client like Excel to authenticate using organization credentials.

See documentation here and here.

Added a unit test in test_single_tenant.py to cover this scenario.

Implementation of #232

[davidhuser]

LGTM, although I would prefer also to assert for the header value in other tests as well (i.e. when there is no authorization_url).

[romaindup]

Thanks for reviewing. Fair ask for the other tests. I've added a header validation for the 401 responses.

[davidhuser]

LGTM 👍

[romaindup]

Anything more required before this can be merged?

[JonasKs]

No, I'll get to it this weekend ☺️

[JonasKs]

Thank you so much!!