[google-auth-library-java] Add support for mTLS workload authentication

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

查看已关闭的 pull request #1149，以及链接的 Workload Identity Federation with X.509 certificates 文档和 AIP-4118，以了解预期的 mTLS 工作负载身份验证行为。当 Java 库能够与这些参考资料保持一致地支持此身份验证流程时，即视为完成；该 issue 未指定具体文件或测试。

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