Allow giving Asteroids a speed and vector

[Hanakocz]

Hello, currently we can spawn asteroids either manually in editor or by script, but they always have default speed and default vector (down). It would be nice to let us modify those values so we can spawn asteroids with specific vectors (so we can make the orbital "random" directions for example). Currently, I haven't found a method on their entity, that would affect them in this regard.

[Bilka]

The create_entity function has a field for velocity for asteroid. I also added LuaEntity::get_movement() and set_movement() for 2.0.67.