This is a simple diff to add a device list type to where it'll be stored and accessed.
With this type this works fine without migration, we could also update the type to deviceList: ?RawDeviceList but then we'll need a migration that reads the data from DB, convert it to objects, adds deviceList: null, and saves it in DB to make it safe - don't have a strong opinion about it.
We want this field to be nullable, this will indicate that we have a new user, and we need to fetch the device list for it from Identity, which will be implemented in ENG-7824. (cc. @inka)