Draft: port to libsoup3 (+ drop deprecated apis, stop using deprecated glib api)

This is continuing the work started in !28 (closed). As of right now i've only rebased the existing work on the lastest commit and tested the changes locally. Everything builds properly and seems to be working correctly. Check !28 (closed) for more details of what's included in the MR.

Further discussion is requrired before adding anything new to this MR.