chore: add ProGuard rules for this library

[Goooler]

Add ProGuard/R8 keep rules to ensure this lib will be compatible with obfuscation enabled, the files under src/main/resources/META-INF/proguard should be interpreted by R8 automatically.

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[Goooler]

Actually, we can just keep a few classes under com.google.api.client.auth instead of all of them, but I have no more time to investigate that.