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

This allows having constraints on the pw properties and the GObject
properties of the proxy, instead of just on the global properties.

This is only useful for constraints on impl proxies, since the globals
coming from the registry don't have a proxy object associated at the
time they are added in the object managers

... and use pw_properties_update() underneath

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 allows activating a dummy item for unit testing

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.