+ add a ports-changed signal

+ better logs by default
+ allow loading modules in tests

This is a simpler endpoint implementation that does not require
any node setup. It should be good to use with test nodes (fakesrc/fakesink),
V4L2 nodes, and maybe others in the future.

Implementations are not required to provide implementations for these
if they don't need them

+ 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