mod_s2s: Add event where resolver for s2sout can be tweaked

Could be used to implement custom connection methods (c.f. mod_onions)
without needing to duplicate the rest of route_to_new_session().

Adds a feature to enable detection since it can be difficult to detect
support for an event otherwise.

core/features.lua

@@ -4,5 +4,7 @@ return {
 	available = set.new{
 		-- mod_bookmarks bundled
 		"mod_bookmarks";
+
+		"s2sout-pre-connect-event";
 	};
 };

plugins/mod_s2s.lua

@@ -230,6 +230,10 @@ function route_to_new_session(event)
 			resolver;
 		});
 	end
+
+	local pre_event = { session = host_session; resolver = resolver };
+	module:context(from_host):fire_event("s2sout-pre-connect", pre_event);
+	resolver = pre_event.resolver;
 	connect(resolver, listener, nil, { session = host_session });
 	m_initiated_connections:with_labels(from_host):add(1)
 	return true;