This module allows wireplumber to create static nodes that match a specific
device using a spa node factory. Matching is optional, and if there is no match,
the node will always be created.

This ensures that endpoints with keep=false will still have
a chance to link when ones with keep=true exist. This also effectively
means that role priority does not matter when keep=true: we keep these
links active no matter what, while policy still applies normally for
all others.

For example, a final sorted list with these endpoints will end up
looking like this:

* ep1, role priority=25, keep=false
* ep2, role priority=20, keep=false
* ep3, role priority=25, keep=true
* ep4, role priority=75, keep=true

... which will effectively cause ep1, ep3 and ep4 to be linked.

keep=true should work in both ways:
* keep the endpoint with this property linked at all times
* keep other already linked endpoints when linking this one

This is no longer needed since we sort endpoints by role priority
before trying to link them and we link only the highest priority one.
After this sorting, the highest priority endpoint is guaranteed
to be able to link, so _can_link_stream() always returns TRUE.

The proxy takes more roundtrips to get updated and this makes the
policy fail to find a default endpoint at startup, right after
the devices have been discovered

This also removes the weak ref on the session, since the object
manager internally maintains a reference on the objects that
it manages.