sbus_futaba: add decoder for SBUS hobby remote control by Futaba

sbus_futaba: add decoder for SBUS hobby remote control by Futaba

This is the SBUS remote control by Futaba, the 25 bytes on top of UART.
Not the computer peripheral bus. Hence the suffix in the decoder name.

The implementation was tested with synthetic data. Example captures with
real world data have yet to become available. This implementation shows
the message framing, the proportional and digital channels' values, and
the flags. Several warnings for short and long and invalid messages are
implemented, as are user adjustable channel value range limits.

The boilerplate may need adjustment. All naming was made up by me based
on what information was available (vendor doc was missing).