Support for authenticating via GitHub OIDC

[manicminer]

• Add a new authorizer GitHubOIDCAuthorizer which supports OIDC token exchange for authenticating to Azure Active Directory

The GHA workflow that tests this runs on a hosted runner and is already committed to main, for testing this PR, and also includes a step that runs the azure/login action for validating that the same backend configuration is valid for the GitHubOIDCAuthorizer test. The azure/login action is not needed for this support to work, it's only being run for comparison.

Docs References:

• https://docs.microsoft.com/en-us/azure/active-directory/develop/workload-identity-federation
• https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-oauth2-client-creds-grant-flow#third-case-access-token-request-with-a-federated-credential
• https://docs.microsoft.com/en-us/azure/developer/github/connect-from-azure?tabs=azure-portal%2Clinux
• https://docs.github.com/en/actions/deployment/security-hardening-your-deployments/configuring-openid-connect-in-azure

[simongottschlag]

I think the audience required by Azure is missing

[manicminer]

Thanks @simongottschlag! I was staring at this for too long and my brain could not do the diff :D

[jackofallops]

LGTM 🚀