migrate existing logged warnings to slf4j logs

[zhumin8]

Debug logging are introduced in https://github.com/googleapis/sdk-platform-java/pull/3403. We have existing undocumented log entries (mostly for warnings) that needs to be migrated.

• Refactor exposed logging classes, to improve readability and make it easier to maintain. context: https://github.com/googleapis/sdk-platform-java/pull/3403/files#r1964339761
• Expose a generic logging method that can be used for more general purpose logging, so that we can migrate existing warnings to it.
• Analyse and migrate existing logs (this should cover both auth and gax. determine if google-cloud-java is also in scope for migration)