add param_map_rc message

@LorenzMeier 
This is a proposal for a message which ask the system to map/bind a rc channel to a parameter.

When the rc channel changes the system should update the parameter value. This feature allows to use a potentiometer knob for parameter tuning.

PR for px4 and qgc will follow.
@LorenzMeier this is for you

For the new message I just copied the param_id line from the existing param messages