lib: Introduce WpSettings API

- WpSettings is a singleton object which attaches itself to the core
  and registry, it provides a get_instance () for its clients.
- WpSettings provides API to get/set wireplumber settings and rules.
- main.c loads the new object and makes sure it is available for
  for all the modules and scripts. This is achieved by introducing
  a new activation step.
- Add the lua bindings for get_setting API.

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

@@ -210,4 +210,6 @@ SANDBOX_EXPORT = {
   State = WpState_new,
   LocalModule = WpImplModule_new,
   ImplMetadata = WpImplMetadata_new,
+  Settings = WpSettings,
+  I18n = I18n
 }