feat: Implement X509 certificate provider

[nbayati]

This commit introduces the foundational components for workload identity mTLS authentication. It includes classes to load the certificate and private key file paths from a JSON configuration and to provide these credentials as a KeyStore.

This lays the groundwork for future implementation of trust chain handling and subject token generation.

[lqiu96]

Would you be able to take a look at the Sonar report: https://sonarcloud.io/project/issues?issueStatuses=OPEN%2CCONFIRMED&pullRequest=1721&id=googleapis_google-auth-library-java and address some of the easier to fix issue?

Sonar can't run from forks so it wasn't something that came up in the previous review. Some of these look to be quick wins for code quality and we can leave the harder stuff as action items in the future.