Allow plugin url in .tool-versions

### Is your feature request related to a problem? Please describe

If a asdf plugin is broken due to a changed tool URL, while a fix PR is pending on the original archive I often fork and fix the issue in my own issue. While using my own fixed asdf plugin, I have to manually install this plugin, which is awkward for my CI system.

### Describe the proposed solution

I would like to see the option of specifying a plugin URL in the .tool-versions file that can be used to override the URL of a particular plugin in the plugin repository. An "adsf plugin-add " command should check for this value in preference to the default repo value

### Describe similar `asdf` features and why they are not sufficient

Manually scripting a asdf plugin add, which is a different workflow when adding a default asdf plugin

### Describe other workarounds you've considered

Manually specifying every plugin URL rather then take advantage of the URLs in the

Start by reading how .tool-versions is parsed and how the asdf plugin-add command resolves a plugin URL. Confirm the expected precedence between a URL in .tool-versions and the default plugin repository value, then verify that adding a plugin through the normal workflow uses the selected URL.

Written by the indexing model from the issue text.