Allows using the endpoint even if a target is not set.

Keeps consistency with PipeWire

This is to provide easy access to default nodes from the scripts

Do not use WpState at all when use-persistent-storage=FALSE; keep all
the configured default nodes in memory and forget them upon restart

* default endpoints no longer make sense
* follow upstream logic of configured vs effective default nodes
* store node names in json inside the metadata, as upstream does
* add default video source

It looks nicer to have groups of features in 'Features.'
instead of 'Feature.'

It is not needed as si-standard-link can be configured with port contexts

The get_endpoint_acquisition() vmethod has also been moved to WpSiPortInfo, and
therefore renamed to get_acquisition().

Allows checking if the endpoints are still valid before releasing the
acquisition. Sometimes the endpoints are destroyed before the link, and
releasing acquisition is not needed. There is no way to check that with raw
pointers.

Because it is only meant to be used with audio nodes

Because it is only meant to be used with audio nodes

Allows using object managers to listen for registered items

This adds WP_SESSION_ITEM_FEATURE_ACTIVE and WP_SESSION_ITEM_FEATURE_EXPORTED
features, so _activate and _export APIs have been removed. Modules and unit
tests have also been updated.

Add all that goes with it.

Avoids unneeded assertion error messages.