Support extradata in serial continuous

[llewelld]

This is a counterpart to #8 (which is for the state machine).

MessageServiceReauth now supports the sending of extradata from the service to the pico. Previously extradata was only supported from the pico to the service (in the MessagePicoReauth message).

The continuous_cycle_start() and continuous_reauth() functions need to be extended to allow extraData to be sent. Contrariwise, continuous_cycle_start_pico() and continuous_reauth_pico() need to be extended to allow returnedStoredData to be returned.

[llewelld]

Branch created for this: https://github.com/mypico/libpico/tree/support_extradata_in_serial_continuous