Indicate soup API version in rest.pc, rest-extras.pc
Because librest-1.0 allows building against both libsoup 2 and libsoup 3, it ought to indicate which using a soupapiversion
pkg-config variable. This is what libgweather and geocode-glib are doing. This will allow dependencies of librest to fail at build time when they detect an incompatible version of libsoup, instead of failing later at runtime, after a broken package has already been pushed to users.
(Alternatively, you could avoid this confusion by making librest-1.0 hard depend on libsoup 3.)