config: add an example of split-instance configuration

This configuration allows splitting wireplumber into multiple instances.
The "main" instance takes care of loading and managing devices
The "policy" instance does all the linking and implements endpoints
The "bluetooth" instance runs the bluetooth devices & nodes

Test with (in separate terminals):
./wp-uninstalled.sh -c config-split -- wireplumber -c main.conf
./wp-uninstalled.sh -c config-split -- wireplumber -c policy.conf
./wp-uninstalled.sh -c config-split -- wireplumber -c bluetooth.conf