Colomban Wendling

Note that writing an async version of the feature, while reasonably easy, is a little bit more work.

See #1340 if wanted.

This PR won (IMO), so it's the one to review @frlan

I rebased this on master now #1342 is merged, as it has CI infrastructure changes for building with libsoup3. Apart from that it's unchanged from the previous version.

@frlan I think this should really land (or be rejected if need be) before 2.1 so we have libsoup3 support for every relevant plugin.

@frlan IMO we *need* this for 2.1, see https://github.com/geany/geany-plugins/pull/1336#issuecomment-2171361614. I'll merge this in a few days unless somebody complains.