Add configurable auto-format workflow

Now that you can enable `configlet fmt` through `.github/org-wide-files-config.toml`, it would be convenient to also be able to add a synced workflow that allows people to trigger formatting through PR comments along the lines of `/format` in the old v3 repo.

E.g.

```yaml
[configlet]
fmt = true
fmt-command = "/configlet-fmt"
```

or

```yaml
[configlet]
fmt = true
fmt-command = true
```

---

Some issues:

- Formatting requires a deploy key to be set up, so there'd be some initial manual labour involved. This could be achieved by posting an automated message pinging @iHiD whenever the workflow is first added. (Though that might annoy him)
- Depending on the command, there might be overlap with `/format`. Though perhaps some people would want it to overlap if they have two compatible formatters set up. Making the command configurable like in the first example above would solve this, at the cost of complicating the workflow and losing cross-track consistency.