Change base image cache location on mac/windows, add methods for getting XDG config location

[TadCordle]

Fixes #2216.

Adds methods for retrieving the config directory:

• Linux: $HOME/.config/google-cloud-tools-java/jib/
• Mac: $HOME/Library/Preferences/Google/Jib/
• Windows: %LOCALAPPDATA%\Google\Jib\Config

Also changes the cache directory on Mac/Windows to the following:

Before:

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

After:

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

Linux is still at $HOME/.cache/google-cloud-tools-java/jib/. Will need to update documentation to help users migrate the old cache over to the new location.

[chanseokoh]

If we have to do this, should we also change the Linux directory to be consistent, from .../google-cloud-tools-java/jib to .../google/jib? The majority of Java developers are on Windows (in generally, may not be true for developers building images), so may not be a big deal. And if so, maybe we should just go with ../google/jib in all lower cases for both Windows and Mac? But I'm fine in either way.

[TadCordle]

Consistency would certainly make this easier. I don't really care where it ends up either way personally. That said, it sounds like there are some conventions people like to follow on different platforms (see #2211 (review))

[chanseokoh]

Ah, okay. Forget changing google-cloud-tools-java/jib then.