cli.add_forge_now: Allow queue name prefix override

This will allow to schedule add-forge-now requests to different queues. For example [1]
will send git tasks to the add_forge_now_slow:swh.loader.git... queue.

[1]
```
swh ... add-forge-now ... \
  --preset $ENVIRONMENT \
  schedule-first-visits \
    --type-name git \
    --prefix-queue add_forge_now_slow
```