[google-auth-library-java] Auth: Refactor X509Provider

[nbayati]

Refactor X509Provider to move getCertificatePath() and getWorkloadCertificateConfiguration() to a utility class along with their helper methods.

[vverman]

@nbayati I believe this issue can be closed out! It appears I don't have the permissions to close it.

[vverman]

This issue is addressed in this PR -> googleapis/google-auth-library-java#1907

Closing this out.