Support XDG spec for providers cache dir

[tony-sol]

What does this do / why do we need it?

Allow to place providers cache by XDG spec, like ${XDG_CACHE_HOME}/m1-terraform-provider-helper/ with fallback to current default ${HOME}/.m1-terraform-provider-helper/

How this PR fixes the problem?

On app configuration init, searching for existing directory via xdg.SearchCacheFile and if ${XDG_CACHE_HOME}/m1-terraform-provider-helper/ not found, fallback to ${HOME}/.m1-terraform-provider-helper/

What should your reviewer look out for in this PR?

Terraform itself still doesn't support XDG spec, so xdg.SearchDataFile("terraform") probably will always return error, but this allows to use custom terraform build or tofu.

Check lists

• [x] Test passed
• [x] Coding style (indentation, etc)

Additional Comments (if any)

{Please write here}

Which issue(s) does this PR fix?