Allows implementing device specific endpoint creation logic, which is currently
needed for bluetooth endpoints as they need to be created differenly.

there are underlying issues with the state management of the graph,
so it's not a very good idea to rely on it to activate / deactivate
our convert node depending on the links that exist

instead, track the links ourselves and create/destroy the corresponding
links to the target node accordingly; it's a more robust approach

it was also not entirely correct that we would previously configure
the convert node to be a driver (node.driver=true); maybe this caused
the underlying issues in the first place... we don't need it now
anyway, so it's gone

Just like media-session does
JACK nodes should be left untouched

1. device export proxies must be destroyed manually since they are
   not associated with the WpRegistry
2. the monitors should not disconnect before all WpSpaDevice objects
   are destroyed; remove the manual disconnect call and let GObject
   ref counting do its job (the core will disconnect when its last ref
   count is dropped after the last monitor plugin is destroyed)

* do not copy the full alsa node properties set
* use a node description that makes the nodes look better in JACK
* use . instead of / as a separator for the node.name, like elsewhere
* add audio.convert spa lib association in the tests that use si-convert;
  previously it used to work because library.name was present in the
  properties copied from the adapter (and it so happens that the adapter
  lives in the audioconvert spa plugin as well ...)

JACK uses : as a separator to distinguish the port name
from the node name, so it ends up doing wrong separations
if we have : in the node name

This may happen if the plugins were loaded but not activated
due to some parse error in wireplumber.conf

previously, we would only remove the endpoint associated with
the node, but not the monitor endpoint...

to implement that, we now store the node's associated session items
in the node's qdata instead of a local hash table, so that we
can store an arbitrary ammount of session items per node

there is no need to add self in the transition's data,
it's already in source_object

This fixes a failure to activate the monitors if we are using
si-adapter directly instead of si-audio-softdsp-endpoint

Requires PipeWire < 0.3.3 or > 0.3.6 to actually work

In case we have a multi-channel capture device and a mono client,
treat the MONO port as UNKNOWN and let it link with the first
port available on the device

Easy to test with arecord, which by default captures in MONO

this allows slightly easier testing while every client uses node.target
but it's easier to find the endpoint id than the node id
through `wpctl status`

We have multiple instances of the monitor plugin, but that's ok.
connect/disonnect will not do anything bad if called multiple times

We need to connect later so that the first connection is the one from main().
Otherwise, if there is a connection error, we will see the warning from the
monitor first.