[google-auth-library-java] Serialization Issues: Auth Library transient keyword usage

There have a few reports of the transient keyword resulting in Null Pointer Exceptions (NPE) when serializing credentials.

This ticket is to track the reports and see how much this is impacting users. We may need to investigate the usage of serializing the Credentials and certain transient fields.

Issues:
- https://github.com/googleapis/google-auth-library-java/issues/1538
- https://github.com/googleapis/google-auth-library-java/issues/1347
- b/416096444
- https://github.com/googleapis/google-cloud-java/issues/14050

Artifact/ Proof of the Issue(s):
- https://github.com/googleapis/google-auth-library-java/pull/1772 - Calendar is transient and leads to a NPE when serializing