Allow customizing the path of the global .tool-versions file

[pgilad]

Is your feature request related to a problem? Please describe

The current solution always clutters the user homedir with the .tool-versions (or name override) file

Describe the proposed solution

I would like to have the ability to explicitly set the global .tool-versions alongside with it's path (would usually resort to ~/.config/asdf/.tool-versions myself).
Alternatively - one can just state the base directory for the global .tool-versions file, and then it will allow the ability to use the base path alongside the ASDF_DEFAULT_TOOL_VERSIONS_FILENAME override variable.

Describe similar asdf features and why they are not sufficient

None relevant

Describe other workarounds you've considered

None relevant

[hyperupcall]

If the problem is that .tool-versions automatically clutters the home directory, would you also consider #1351 a fix? Instead of having to set a variable, you can just put the file in one of the asdf base desktop spec directories.

[pgilad]

#1351 doesn't handle the global .tool-versions. You need to modify https://github.com/asdf-vm/asdf/blob/master/lib/functions/versions.bash#LL23C18-L23C27 for that