Change cache location on Mac/Windows

[TadCordle]

Currently, the base image cache is located at:

• Mac: $HOME/Library/Application Support/google-cloud-tools-java/jib/
• Win: %LOCALAPPDATA/google-cloud-tools-java/jib/

It may be a good idea to change these to:

• Mac: $HOME/Library/Caches/Google/Jib/
• Win: %LOCALAPPDATA/Google/Jib/Cache/

We may need to come up with a migration strategy so we don't break users who rely heavily on the cache (e.g. offline users).

Checklist:

• Code (Change base image cache location on mac/windows, add methods for getting XDG config location #2218)
• Docs (Update README for new cache location #2231)

[briandealwis]

The Win cache should be in %LOCALAPPDATA%/Google/Jib/Cache just to be clear that it can be removed without affecting other possible configuration files.

[chanseokoh]

We may need to come up with a migration strategy so we don't break users who rely heavily on the cache (e.g. offline users).

I'm leaning toward documentation rather than adding code to do automatic migration for a number of reasons.

[TadCordle]

Gonna keep this open until the documentation is updated

[chanseokoh]

Alos, does it make sense to update CHANGELOG to describe the changes to the base image cache directory on some platforms by #2218?

[TadCordle]

Oh right, was gonna do that but forgot.