libmavconn: add MAVLink v2 signing support

[vooon]

Summary

This PR adds initial MAVLink v2 packet signing support in libmavconn and refactors status handling to cleanly separate TX and RX state.

What changed

• Added MAVLink signing API to MAVConnInterface:
  • setup_signing(...)
  • disable_signing()
  • set_accept_unsigned_callback(...)
• Added internal signing state wiring:
  • dedicated TX signing state
  • dedicated RX signing state
  • RX signing stream replay-tracking state
• Split TX/RX mavlink_status_t usage:
  • TX finalization now uses dedicated TX status
  • RX parser and RX status use dedicated RX members
• Added signing-focused UDP tests:
  • signed packets are accepted with matching key
  • bad signatures are reported when keys mismatch
  • unsigned packets can be accepted via callback override

Files touched

• libmavconn/include/mavconn/interface.hpp
• libmavconn/src/interface.cpp
• libmavconn/test/test_mavconn.cpp

Validation

Ran from /ws:

• source /opt/ros/kilted/setup.bash
• colcon build --packages-select libmavconn
• colcon test --packages-select libmavconn
• colcon test-result --verbose

Result:

• Build passed
• Tests passed (0 errors, 0 failures)

Notes

• Signing timestamp default uses MAVLink signing epoch semantics (2015-01-01, 10 µs units).
• Feature is implemented at libmavconn transport layer; higher-level configuration/UI wiring can be added in follow-up PRs.