[Feature]: Add message to set the trim values

[PBechon]

The Flight ControlState API allow setting the values for the trim and there is currently no way to set them from the SimpitAPI.

One solution would be to reuse the Rotation message and use the unused bits from the bitmask to indicate that the values in the message should be the new trim value.

[PBechon]

The trim values does not seem to work with an autopilot such as the one used by Simpit. Setting the trim values using the FlightCtrlState API does not seem to do anything. I tried to add a new channel that control the trim values (with the same code as the one controlling the rotation, only updating the trim values of the FlightCtrlState).

kOS seems to have the same issue. See here for instance.

Not sure how to proceed from here, except excluding this from the feature we want to support.

[PBechon]

For those that want to try, see the diff file of what I tried.

Following a discussion on Discord, adding trim can be done in the controller without using KSP builin trim. It seems also that KSP built-in trim are not used when setting rotation command the way we use them, meaning that the controller command will in any way ignore KSP trim. So I suggest not to implement a message to set KSP trim values with Simpit and instead favor a trim setting Arduino side, by each user.