Mavlink FTP: List and Download commands working end to end

FYI: I switched back to using a Nak:kErrEOF to signal end of read loops for List and Read commands. Other options overcomplicated things both on the QGC and PX4Firmware side.

This has List, Open, Read and Terminate fully working. Requires a PX4Firmware pull which I also have to get it all to work. Still have a list of work items against this, but it is mostly additional hardening and more comprehensive unit testing at this point.

Next steps:
- PX4Firmware pull to get things working from that side
- More hardening/testing of these commands
- Better UI
- Move on to remaining commands