Add MAVFTP parameter and mission support

[RyanCraighead]

Summary

• Adds a MAVLink FTP manager for ArduPilot virtual file transfers over FILE_TRANSFER_PROTOCOL.
• Downloads ArduPilot parameters via @PARAM/param.pck?withdefaults=1 and parses the packed parameter file into the existing parameter update path.
• Uploads locally changed ArduPilot parameters via @PARAM/param.pck from the existing Write (ROM) path, matching ArduPilot's save-on-close behavior for parameter FTP uploads.
• Downloads/uploads missions via @MISSION/mission.dat using ArduPilot's MISSION_ITEM_INT file format.
• Falls back to existing MAVLink parameter and mission protocols when MAVFTP is unavailable, times out, or returns invalid data.
• Registers the new source/header files in both qmake and CMake build lists.
• Adds focused CMake unit coverage for the MAVFTP parameter file, mission file, and FTP payload helpers.

Validation

• Ran git diff --check origin/master..HEAD.
• Added and ran mavftp_unit_tests covering packed parameter parsing, parameter upload encoding, mission.dat parsing/writing, malformed file rejection, MAVFTP opcodes/offsets, ACK/NAK/EOF handling, sequence wrap, and payload-size limits.
• Checked the implementation against current ArduPilot @PARAM/param.pck, @MISSION/mission.dat, and MAVFTP response handling.
• Confirmed Linux qmake build, CMake/Ninja build, and ctest passed on the fork validation branch: https://github.com/RyanCraighead/apm_planner/actions/runs/25715278364. The final PR branch excludes the temporary validation workflow.
• Ran local CMake configure with Visual Studio; it stops at Qt5 discovery because Qt5 is not installed on this machine.

Relates #1252

Transparent note: this implementation was assisted by OpenAI Codex.