Non-language plugins

Rbenv allows you to install plugins that hook into more functionality of the version manager. For example, rbenv-vars allows you to set project specific environment variables and are automatically loaded when Ruby is executed.

I've replicated this for asdf called [asdf-vars](https://github.com/excid3/asdf-vars), but right now I have to inject into the asdf-exec script to add this functionality. The downside is that this is going to cause a conflict when updating asdf in the future.

There's a list of other interesting plugins here: https://github.com/rbenv/rbenv/wiki/Plugins

These are the hooks that they support for plugins: https://github.com/rbenv/rbenv/wiki/Authoring-plugins#rbenv-hooks

Would adding hooks to support non-language plugins to asdf be something that would get accepted as a PR or is already on the roadmap?

The issue points to the asdf-exec script, asdf-vars, and the linked rbenv plugin and hook documentation; start by reviewing those references and how plugins are currently integrated. Define the supported non-language hook behavior and its completion criteria before proposing a PR that avoids edits injected into asdf-exec.

Written by the indexing model from the issue text.