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.

the global is stored internally and the returned ref is only useful
in the WpProxy code, not in the registry_global() event

it doesn't matter, because we don't use it, but for the sake of readability

some code is expecting things to happen in that order...

... in case the global is removed from the registry before
the initial augment completes

When a new global is created, it is not certain
if the registry global event or the proxy bound event will
be fired first. In order to make sure we associate all
proxies to their WpGlobals correctly, we now wait a core sync
before exposing globals to the object managers, so that in case
the implementation proxy receives the bound event after the
registry creates the WpGlobal, we can make sure to use this
proxy instead of constructing a new one through the object managers

There are 3 kinds of WpProxy objects:
 * the ones that are created as a result of binding a global
   from the registry
 * the ones that are created as a result of calling into a remote
   factory (wp_node_new_from_factory, etc...)
 * the ones that are a local implementation of an object
   (WpImplNode, etc...) and are exported

Previously the object manager was only able to track the first kind.
With these changes we can now also have globals associated with
WpProxies that were created earlier (and caused the creation of the global).
This saves some resources and reduces round-trips (in case client
code wants to change properties of an object that is locally
implemented, it shouldn't need to do a round-trip through the server)