Add something similar to speed_multiplier_when_out_of_energy for spidertrons

[MrSmoothieHuman]

This is in relation to another forum post, being this one here: viewtopic.php?t=128126 - where spidertrons would move slowly when out of fuel instead of just completely slowing to a stop, and i'd really like to this stay as an optional feature for spidertrons to use
I imagine it to work similar to flying robots, where they have an optional parameter that slows them down by a settable amount (and obviously, having it at 0 would make spidertrons work how they currently do)

(While writing this i also thought about having a parameter for how long it takes a spidertron to reach its minimum set speed, instead of having it be set to 10 seconds - although this is out of the scope of the original request, and im not sure how feasible either of these are; but i wanted to mention it nontheless )