Support Bearer authentication for OAuth token requests

[m-radmacher]

If you enable certificate authentication instead of token authentication in Microsoft Entra ID, you must authenticate via a Bearer token. (see https://learn.microsoft.com/en-us/answers/questions/346048/how-to-get-access-token-from-client-certificate-ca) This is currently not supported by oslo.

I have already begun implementing this option and it seems like a trivial change (https://github.com/pilcrowOnPaper/oslo/compare/main...m-radmacher:oslo:feat/oauth-jwt-authentication).

If you're interested in merging this I would probably need to update the docs further.

[pilcrowonpaper]

Closing this as out-of-scope - Oslo will only support authorization code grant type for OAuth