Add support for mTLS workload authentication

[stanczykj]

There is a new way of authenticating in Workload Identity Federation using mTLS (currently in pre-GA stage), that this library doesn't support. There was a pull request with this feature a while back, but for some reason got closed https://github.com/googleapis/google-auth-library-java/pull/1149

Links: https://cloud.google.com/iam/docs/workload-identity-federation-with-x509-certificates https://google.aip.dev/auth/4118

[lqiu96]

@lsirac I think you would have the most knowledge about the status of this FR.