feat: async serial

[roby2014]

Tried following the other connection wrappers structures.
Needs testing (i'll test soon).

[roby2014]

Did a simple test with serial:/dev/ttyACM1:115200:

// ...
        let mut connection =
            mavlink::connect_async::<mavlink::ardupilotmega::MavMessage>("serial:/dev/ttyACM1:115200").await?;
        connection.set_protocol_version(self.mavlink_version);
// ...
            select! {
                res = connection.recv() => {
                    match res {
                        Ok((header, msg)) => {
                            trace!(?header, ?msg, "received message from connection");
                            //...
                        }
                        Err(e) => {
                            error!("failed to read: {e}");
                        }
                    }
                }
                res = channel.recv() => {
                    // ...
                }
            }

and I'm able to get data:

received message from connection header=MavHeader { system_id: 1, component_id: 1, sequence: 133 } msg=ATTITUDE(ATTITUDE_DATA { time_boot_ms: 389342, roll: 0.0, pitch: 0.0, yaw: 0.0, rollspeed: 0.0, pitchspeed: 0.0, yawspeed: 0.0 })

received message from connection header=MavHeader { system_id: 1, component_id: 1, sequence: 134 } msg=VFR_HUD(VFR_HUD_DATA { airspeed: 0.0, groundspeed: 0.0, alt: 0.0, climb: 0.0, heading: 0, throttle: 0 })

[peterkrull]

I would also like to have async serial be upstreamed so there is consistency between sync/async. Though looking at your code, is it not problem that the port is locked behind a single mutex, i.e. no reader and writer half? If I were to wrap the AsyncSerialConnection in an Arc and create a reader and writer thread, would the reader not be able to lock out the writer indefinitely if there are no incoming messages?