Add SoupSession:proxy-resolver

Add SoupSession:proxy-resolver

Now that GSocketClient lets you override its proxy resolver, allow
setting a GProxyResolver on a SoupSession, to eventually pass on to
the GSocketClient.

(In the interest of not breaking things this late in the release
cycle, all of the old SoupProxyURIResolver code still exists as well,
in parallel, but in 2.43 it will be removed and replaced with
GProxyResolver-based backward-compat stuff.)

https://bugzilla.gnome.org/show_bug.cgi?id=680273