Combine .tool-versions based on other project's .tool-versions files

What does this Merge Request do and why?

This adds a support script (support/asdf-combine) which combines our .tool-versions with the ones from upstream projects.

Unfortunately we need to move the current gdk .tool-versions to a template file.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.