Support initial values in Pairing

[drf]

Pairing already supports delivering an initial introspection. When this is done, it should also be possible to set initial values on all interfaces.

As an example, suppose we're registering a device with an interface com.example.Info in its initial introspection. A pairing API call should support something like:

[...]
"initial_data": {
    "com.example.Info": {
         "serial": "1491414"
    }
}
[...]

After registering the device, calling AppEngine API on the /com.example.Info/serial endpoint should return the value set, even if the device never connected.