Port to libsoup3

While libsoup 3.x is not yet released and won't be for a couple more months, I'm starting to port projects using it over to the new API, so that the ecosystem can drop the old version as soon as possible. I used gvfs as a starting point for that.

The code is probably fairly rough right now, but basic testing shows that it is functional (at least the things I tested, which is file operations on DAV shares, as well as the HTTP backend).

I'll appreciate any review comments.