Add AP_CRSF_Protocol class with shared enums and structures that will
be used by both AP_RCProtocol_CRSF and AP_CRSF_Telem.
Includes:
- FrameType, CommandID, DeviceAddress, CommandGeneral, CommandVTX,
CommandRX, CustomTelemSubTypeID, ProtocolType enums
- Frame, SubsetChannelsFrame, LinkStatisticsTXFrame structs
- HeartbeatFrame, CommandFrame, ParameterPingFrame,
ParameterDeviceInfoFrame structs
- CRSF_MAX_CHANNELS, CRSF_FRAMELEN_MAX, CRSF_HEADER_LEN,
CRSF_FRAME_PAYLOAD_MAX defines
- get_frame_type() debug helper method
