This is an asynchronous operation class, like GTask,
but it is made to execute several operations underneath,
using a state machine, instead of just a single operation.

and in case it's missing, also #define some things to make compilation work,
since this is only available in a pipewire branch right now

This will solve a possible race condition in the unit test when destroying the
autotestsrc proxy while its node_event_info is being called.

the global is stored internally and the returned ref is only useful
in the WpProxy code, not in the registry_global() event

it doesn't matter, because we don't use it, but for the sake of readability