Allow to triggering forges listing with oneshot policy and some priority tasks.
Thus ingesting those faster and without manual intervention as we currently do
[1]
Example use case for now is for small forges:
```
swh lister list --lister gitlab \
--priority high \
--policy oneshot \
--db-url postgresql://postgres@localhost:5432/swh-listers \
api_baseurl=https://gitlab.ow2.org/api/v4/
```
Related T1919
[1] along the lines of psql update "the listed tasks with priority to high", then wait for it to finish somehow, and then at some point, remove that priority with psql update yet again... tedious!