Investigate the use of Serializable in the Auth Library

[lqiu96]

From a discussion here: https://github.com/googleapis/google-auth-library-java/issues/1538#issuecomment-2490440601

Certain third party dependencies are serializing and de-serializing the Auth Credentials. This seems to have been causing some issues reported by users.

Potential action items from this ticket:

• Figure out the context of why Credentials are implementing the Serializable interface
• Determine if we want to continue to support Serializable (If so, how to make all Credentials Serializable. If not, can we remove Serializable without a breaking change?)
• Add documentation or warning to the README about Serializable