lorawan: add callback for datarate changes

Add a callback to notify applications when the datarate has changed.
This allows applications to know when payload sizes have changed without
needing to call `lorawan_get_payload_sizes` before each transmission.

This also enables:
 * Monitoring of network conditions on the device
 * Determining if a network connection has been lost

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>