Add `--quiet` parameter to `asdf install`

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

As part of our normal setup scripts we tend to add `asdf install | sed '/is already installed/d'` so we can reduce the terminal noise when nothing needs to be done.

### Describe the proposed solution

I would like to see a `-q`/`--quiet` parameter being added to `asdf install`, or simply a `ASDF_QUIET=1` / `QUIET=1` env variable I can set in our setup scripts that would work for all commands.

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

None.

### Describe other workarounds you've considered

Adding a sed pipe where necessary.
```
asdf install | sed '/is already installed/d'
```

Start at the implementation of the `asdf install` command and trace how its output is produced when versions are already installed. Decide whether the accepted interface is a `-q`/`--quiet` flag or an environment variable, then verify that setup scripts can suppress the unwanted output without changing normal installation behavior.

Written by the indexing model from the issue text.