mavftp: make usable and useful

- removed unused MAVProxy imports (no longer dependent on MAVProxy)
- added `FTPUser` class for interacting conveniently via mavftp
   - added `list` method, which returns the requested files list
   - added `get` method, which gets and saves a requested file
   - added 'params' method, which gets, parses, and saves a parameter file
- added CLI to access all available commands
- used some f-strings, and `subparsers.add_parser(required)`, so requires Python >= 3.7