Support MissionItems for LAND, TAKEOFF

[hamishwillee]

We need MissionItem functionality for takeoff and landing. These items should attempt to properly support common options for MC, Plane and VTOL (where they differ). Specifically I would hope that we could have a single land and single takeoff item to support all of these:
https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_LAND
https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_TAKEOFF
https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_LAND_LOCAL
https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_TAKEOFF_LOCAL
https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_VTOL_LAND
https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_VTOL_TAKEOFF

Once these exist they also need to be supported in the mission import and download from vehicle and added to tests.

There was some discussion originally that these are not needed because

• MC automatically takes off if no takeoff is present
• Action API supports these functions.

They are needed because:

• MC automatic takeoff does not allow you to specify any takeoff behaviour
• Plane and VTOL do not automatically takeoff if no takeoff item is present
• QC plan import and download from vehicle can end up being different in round trip.

To me this just makes sense.

[julianoes]

MissionRaw can do it all.