[Core] BREAKING CHANGE: Change get_config_dir to honor XDG spec

[HStry]

Description

This commit modifies the get_config_dir function to honor the XDG Base Directory Specification in accordance with https://specifications.freedesktop.org/basedir-spec/latest/, and falls back to a dot-dir in the user's home directory if the relevant environment variables and home directories do not exist.

Testing Guide

On running the az cli, it should now refer to '${XDG_CONFIG_HOME}/azure', or '${HOME}/.config/azure'.

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[yonzhan]

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[HStry]

@microsoft-github-policy-service agree

[bebound]

This is a devastating breaking change.

[HStry]

This is a devastating breaking change.

Yesterday, I indeed found out that it still made the ~/.azure directory after adding this change to my local setup, so yes, as it is, this is definitely breaking.
However it should not be. The XDG spec exists, is backwards compatible, and has been adopted by pretty much all distributions, for a reason. It's rather sad that large projects like Azure appear not to follow it, but I'd need a much longer time to figure out all the locations where this application/module has hard-coded the ~/.azure directory in order to adapt this.

And maybe for backwards-compatibility and consistency, have the first version check if the ~/.azure directory exists, and if so, use that regardless of what the XDG_CONFIG_HOME variable says.