ardupilotmega: update OBSTACLE_DISTANCE_3D message

[rishabsingh3003]

This PR fixes some of the errors I had made before while making the message. Also, I have added a new "orientation" field to the message. It makes it easier for the user to set up a camera that is not facing the centre. I'll do a follow-up PR inside our proximity library to add support for this field.

[peterbarker]

@rishabsingh3003 what's your intent with this? Its getting very, very late to try to get this into the next Copter stable!

Once it's gone out in Copter-stable we're not going to be able to do anything except add extension fields.

[rmackay9]

I think removing the WIP label at least is a good idea and won't affect the format of the message so consumers will be unaffected as well.