`juju wait-for` should grow a `--all` flag

[Simon Déziel]

In the past, a convenient companion snap called juju-wait provided a simple way to wait for everything to go quiet/idle. Now that `juju wait-for` replaced it, it's a bit cumbersome to replicate the old "wait for everything" behavior as one has to use something along those lines:

```
juju wait-for model ci-testing '--query=life=="alive" && status=="available" && len(applications) > 0 && forEach(applications, app => app.status == "active") && len(units) > 0 && forEach(units, unit => unit.workload-status == "active" && unit.agent-status == "idle")'
```

Having something like `juju wait-for --all` (any descriptive flag name would do) would be convenient.

Thanks!