core: Add a "proxy-resolver" property.

Bound to SoupSession's "proxy-resolver" property.

This also bumps the libsoup requirement to 2.42.

This deprecates the following API:
 • GDataService:proxy-uri
 • gdata_service_[get|set]_proxy_uri()
and adds the following new API:
 • GDataService:proxy-resolver
 • gdata_service_[get|set]_proxy_resolver()

Helps: https://bugzilla.gnome.org/show_bug.cgi?id=709758