Support loading plugins from local directories

Is your feature request related to a problem? Please describe

I contribute to a monorepo where we would like to be able to use asdf to manage versions for our tools. We will need to write a number of plugins to be able to support this. asdf currently only provides two options to be able to do this:

1. Put each plugin into its own repository. We do not want to maintain 5-10 new repositories, so we won't be doing this.
2. Put all plugins into a single repository and use some unified scripting to hack in the ability to manage multiple plugins at the same time. This is what Hashicorp does.

Both options are bad options.

Describe the proposed solution

We would like to be able to load plugins from local directories. This would make it possible to have a folder asdf/plugins which includes plugins.

Ideal UX would be something like:

asdf plugin add path/to/local/plugin

Where path/to/local/plugin does NOT point to a Git repository and is simply a folder within the monorepo.

Describe similar asdf features and why they are not sufficient

No similar features.

Describe other workarounds you've considered

As noted, the Hashicorp approach is the best existing workaround and is likely what we will be forced to do in the interim. Still, this forces us to maintain code outside of the monorepo.