This commit enhance `module-default-routes` so that the route properties
(such as volume, channel map...) are saved on modification, and re-applied
each time a particular route is selected.
This commit adds a WirePlumber module which stores the current route for
a given device, and implements the necessary interfaces to restore those
on startup.
The routes are stored as an array of objects in JSON format.
Please note that actually restoring routes on startup is not done by this
module but will be implemented separately in a future commit.
