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.

Start at the `asdf plugin add path/to/local/plugin` entry point and inspect how plugins are currently loaded from repositories. Define the behavior for accepting a plain local directory, including how completion should distinguish it from a Git repository, then verify the command works for plugins stored inside a monorepo.

Written by the indexing model from the issue text.