Port to libsoup3

GNOME 43 will be (maybe) the first version of GNOME to ship with most core apps built against libsoup3, which is ABI incompatible, and can't be mixed with libsoup2 libraries (similarly to GTK2 and GTK3, or GTK3 and GTK4).

The porting tracker is here: libsoup#218

Rhythmbox depends on a number of libraries that depend on libsoup itself, as well as having a large number of libsoup dependencies in its various plugins.

As a start, I'll mention the dependencies I could find:

• grilo
• grilo-plugins
• libdmapsharing

So we can track what work is needed.