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";
 	};
 };