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"the listed tasks with priority to high`,
", then wait for it to finish somehow and at some point, and then at some point, remove that priority with
psql update yet again... tedious!