+ add the useful _find_proxy() method

+ 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

This allows filtering messages without a domain using WIREPLUMBER_DEBUG

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

This extends GLib's logging system, so it is compatible with
g_debug() and friends, but it uses a better logging format and
supports filtering debug domains with wildcards, like in gst.

* It should be possible to activate/deactivate while an item
  is exported (if the item supports it, but that's a subclass matter)
* It should be possible to deactivate without resetting configuration

To ease the use case where we are only interested in the id
and not the proxy object itself

This feature enables the caching of WpEndpointStream objects
inside WpEndpoint, making information about streams readily
accessible to the user of an endpoint