Draft: Generate .tool-versions dynamically

What does this merge request do and why?

This MR aims to generate the .tool-versions file dynamically when needed. I originally went with create a new file called .tool-versions.generated and updated the various asdf <command> calls to be prefixed like ASDF_DEFAULT_TOOL_VERSIONS_FILENAME=.tool-versions.generated but this means you can't simply run asdf <command> as-is outside of the GDK because it will look at .tool-versions by default.

The current problem with the approach in the MR is due to no .tool-versions file existing from a fresh clone, lots of commands immediately fail because there's no directives to say what version of ruby to use etc.

We'll need to keep iterating on this..

cc @nao.hashizume

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1635 (closed)