This won't work with WpSpaPod, obviously...

It's not well-defined; we'll come back to that later

This reverts commit 4e8ae293.

Needed when using audiotestsrc with an adapter

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.

+ 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.

* 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

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)

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