* 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)