[Feature Request] Support multiple global version files

[DerekTBrown]

Is your feature request related to a problem? Please describe

• For CLI tools (i.e. kubectl, istioctl, etc). It doesn't make sense to put these inside a .tool-versions file in a repository, since they should be usable globally.
• If I am developing with a team, I would ideally like to provide everyone a consistent version of these CLI tools. This currently isn't possible on a global level, since this would override users' personal ~/.tool-versions.

Describe the proposed solution

• Similar to other dotfiles, support a ~/.tool-versions.d folder. All files are combined and the latest version is used in each instance.

Describe similar asdf features and why they are not sufficient

• There isn't a direct asdf feature for this.

Describe other workarounds you've considered

• Maintain a script that runs asdf global install X. This is painful, since users have to run it repeatedly.
• Maintain a custom script that generates a ~/.tool-versions. This is also bespoke.