Allow pedestrian routing over highway=platform

[Woazboat]

Please allow pedestrian routing over highway=platform ways. The tag is commonly used instead of highway=footway when there's a bus or tram platform directly on the sidewalk. Currently this leads to broken routing because to the router this looks like an unroutable section in the middle of a footway. It should be considered similar to highway=footway.

Image borrowed from here

See here for the upstream OSRM issue Project-OSRM/osrm-backend#5410
Related Valhalla issue is here valhalla/valhalla#3691

[datendelphin]

Sure, please just add it.