m-lua-scripting: wrap every script in a WpPlugin and expose async activation api

This allows scripts to declare when they have finished their loading,
so we can now also know when wireplumber is done loading and ready to
handle clients

Related to !313

modules/module-lua-scripting/api/api.c

@@ -35,7 +35,8 @@ get_wp_export_core (lua_State *L)
   WpCore *core = NULL;
   lua_pushliteral (L, "wireplumber_export_core");
   lua_gettable (L, LUA_REGISTRYINDEX);
-  core = lua_touserdata (L, -1);
+  if (wplua_isobject (L, -1, WP_TYPE_CORE))
+    core = wplua_toobject (L, -1);
   lua_pop (L, 1);
   return core ? core : get_wp_core(L);
 }