Support client_credentials flow with Microsoft Identity Platform v2.0 using client assertions

[manicminer]

• Commonly referred to as Client Certificate authentication
• I'm not sure if this follows a particular standard, but it's similar to client_credentials flow but signing a JWT for identity instead of sending a shared secret
• Azure AD is the only place I've seen this, so I guess belongs in the microsoft package? Unless there's a standard implementation I missed.

Example/proposal in #464

Documentation

• https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-oauth2-client-creds-grant-flow#second-case-access-token-request-with-a-certificate
• https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-certificate-credentials

[deltamualpha]

I would find this very valuable. @codyoss, are you the current maintainer of this package? The pull request here seems quite reasonable.

[manicminer]

@deltamualpha If you're looking for a maintained implementation of this, I have a package github.com/manicminer/hamilton/auth supporting the various Azure AD authentication methods.