Support external path interface (scripting/ROS/offboard)

[Ryanf55]

Feature request

Is your feature request related to a problem? Please describe.
I have a global path planner that runs offboard. It generates a vector of safe waypoints to achieve a high level navigation goal, such as crossing a mountain range in minimum distance. The path is kinetically feasible. I can't command the plane to follow this path today without using AUTO (or it's not clear how).

Describe the solution you'd like
An ability to feed these waypoints to the plane/copter/rover and have it do look-ahead to not stop at each waypoint.
Describe alternatives you've considered
Setting waypoint radius to zero or using auto, which won't work if the trajectory is created on the fly.

Expectation

If I have a plane or a copter, and I take a large circular path (say, quadruple plan's loiter diameter), discretize it to 100 points offboard, I should be able to feed these
position targets into the vehicle (with lookahead), and it should follow a smooth circle path. Same for copter and plane and rover.

If the points are collinear, both copter and plane and rover should navigate at their cruising speeds and not slow down/circle at each point if desired.

Platform
[x] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Additional context

If you try doing guided waypoints today on plane, it always tries to orbit.
There is currently no way to supply a list of 10 waypoints in GUIDED mode that are all collinear and have the plane fly straight through all them. Copter slows down at each waypoint until you feed it the next one. This requires a companion computer to feed the points at the right moment, which cuts corners and relies on bespoke/precise undocumented timing and distances for each vehicle's navigation controller. This is NOT scalable for generic offboard autonomy.

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