Loosen base loader constructor to accept kwargs

That should allow configuration without convolutions in the sysadm dimension. That is generically add options that may not be understood by all loaders without making them fail.

The ones supporting those options will use it. The others not supporting those will just not use it.

Implementation wise, this should work as most loaders I checked (cvs, bzr, mercurial, svn, arch, archive, aur, ...) are already calling the super() class providing the kwargs to the upper class in their constructor call. While the top level class BaseLoader is not, that's what this very diff changes to allow it (and the task layer refactoring we did a while back is already wired as we want too so nothing to change there!)

@olasd @vsellier what do you think?

@guillaume This is the 2nd "simple" solution i proposed in your mr about the configuration change for the add-forge-now [1]

[1] Refs. swh/infra/ci-cd/swh-charts!62 (closed)