Support custom COMMAND_LONG listeners and be able to ACK

[sanderux]

We are using MAV_CMD_WAYPOINT_USER_1 to run custom commands from a mission
This ability is currently missing from mavsdk making it impossible for us to switch.
Would be nice to be able to add a custom action listener. We would need the param values too

@julianoes FYI

[julianoes]

I think this should be possible using the passthrough plugin. Before I add it as explicit functionality we would have to standardize it in MAVLink. We had several discussions about custom action commands before, the previous attempt was a bit too complex though, in my opinion: mavlink/MAVSDK#1581 (comment).

[sanderux]

User actions are standardized in mavlink no?
What would be wrong with extending the actions call with a 'custom action' handler where you can listen to all commands and choose if you want to handle / ack them?

[sanderux]

Here: mavsdk/action_server.py

[julianoes]

Does the autopilot (e.g. PX4) support sending USER_1 commands and waiting for an ack? That would be the first step.

[sanderux]

Yes it does, we are already using this with pymavlink

[julianoes]

Ok, which PX4 version do I use to test this?