This aligns better with the general design of consuming property
objects on constructors, both in PipeWire and WirePlumber APIs

It can be a bit tricky because the "installed" signal may be fired
from inside the context of wp_core_install_object_manager(),
in which case the main loop should not be executed (or it will never quit)

We can now call wp_proxy_request_destroy() on endpoint links and
the WpImplEndpointLink together with the session item that created
it will be cleaned up

+ better logs by default
+ allow loading modules in tests

+ expose the export transition in the session item class
+ make the export-related flags immutable
+ add an export error flag
+ update and improve documentation

+ replace calling execute_step(..., STEP_ERROR) with rollback
+ implement deactivate internally using rollback

This unifies deactivation steps, which are common between deactivate()
and calling execute_step() with WP_TRANSITION_STEP_ERROR at the
end of a failed activation transition.

including wp/spa-props.c currently re-defines G_LOG_DOMAIN
in the spa-props.c unit test

+ enable the new log writer on the executables
+ enable structured logging in the tests

This allows the implementation to rollback changes, cancel jobs, etc

* introduces API to export session items
* introduces small changes in the WpSiEndpoint & WpSiStream
  interfaces to make it nicer to work with
* ports WpImplEndpoint to use PW_TYPE_INTERFACE_Endpoint
  to export. Depends on:
  https://gitlab.freedesktop.org/pipewire/pipewire/-/merge_requests/246
  (was merged after 0.3.2)

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.