Add rest_proxy_add_soup_feature()

This function can be helpful if one wants more control over libsoup
features than what librest simple API provide. For example, to get full
access to libsoup cookie API (say to be able to add arbitrary cookies to
the soup session), one can do:

RestProxy *proxy = g_object_new(REST_TYPE_PROXY,
                                "url-format", url,
                                "disable-cookies", TRUE,
                                NULL);
SoupSessionFeature *cookie_jar = SOUP_SESSION_FEATURE(soup_cookie_jar_new ());
rest_proxy_add_soup_feature(proxy, cookie_jar);

It's then possible to use all the soup_cookie_* methods to deal with
cookies.

https://bugzilla.gnome.org/show_bug.cgi?id=728340