Move webservice_xmlrpc_client to core

In MDL-56320, we discovered that the webservice_xmlrpc plugin can't be uninstalled as it breaks core features like registration and course publishing. This is because they are using the class webservice_xmlrpc_client which is contained in this plugin.

This seems to break the rule that core components shouldn't depend on plugins, so it would make sense to move this class into core. The class is not actually used by the plugin itself.