Support plugin install giturl#gitref

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

i'm always frustrated with i have to fill these things out.

lets support installing asdf plugins from a git repo with a gitref (tag, branch, commit)

### Describe the proposed solution

```
asdf plugin install https://github.com/heliumbrain/asdf-just#6cc5790
```

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

1. cajole broken plugin author into fixing their code
2. get stonewalled there
3. fork the broken plugin repo
4. fix the code
5. remove old broken plugin
6. point at different repo

lets say the author fixes their code, but then later on decides to change things...

the only way i can guard the onboarding process of my team is to fork every asdf plugin and use those repos instead.

### Describe other workarounds you've considered

crying.

Start at the `asdf plugin install` command entry point and trace how a plugin repository URL is parsed and fetched. Verify how a `#gitref` suffix should select a tag, branch, or commit, and confirm completion by installing the example repository at the requested ref without requiring a fork.

Written by the indexing model from the issue text.