feat: Add incompat flag field to MavHeader

mavlink-core/src/connection/direct_serial.rs

mavlink-core/src/connection/tcp.rs

mavlink-core/src/connection/udp.rs

mavlink-core/src/lib.rs

mavlink/examples/embedded-async-read/src/main.rs

mavlink/examples/embedded/src/main.rs

mavlink/tests/mav_frame_tests.rs

mavlink/tests/test_shared/mod.rs