[Conf] Allow to load users plugins from plugins.d

Issue: #2848

conf/common.conf

@@ -36,4 +36,5 @@ lua = "$RULESDIR/rspamd.lua"
 modules {
   path = "${PLUGINSDIR}";
   fallback_path = "${SHAREDIR}/lua"; # Legacy path
+  try_path = "${LOCAL_CONFDIR}/plugins.d/"; # User plugins
 }