Reinstate task for SvnLoaderFromDumpArchive and add worker entrypoint
requested to merge anlambert/swh-loader-svn:reinstate-load-svn-from-archive-task-plus-improvements into master
There is still cases where subversion repositories have been archived to dump files while their hosting service got discontinued.
So we still need to be able to schedule tasks executing SvnLoaderFromDumpArchive
to ingest such repositories.
Also add support for downloading a dump file.
Related to swh/infra/sysadm-environment#5331.