Allow routing by foot along public_transport=platform or railway=platform or highway=platform

[joshinils]

I'd like to suggest including any public transport platform for foot routing.
Yes, they can technically be closed to the public, but I've never seen that happen in Berlin unless the station itself is out of order, under repair etc.

Examples:
https://www.openstreetmap.org/directions?engine=fossgis_osrm_foot&route=52.59788%2C13.33276%3B52.59589%2C13.33639
https://www.openstreetmap.org/directions?engine=fossgis_osrm_foot&route=52.53626%2C13.34430%3B52.53613%2C13.34314
https://www.openstreetmap.org/directions?engine=fossgis_osrm_foot&route=52.60508%2C13.32340%3B52.60732%2C13.32093

I understand including stairs may not be the best option and potentially lead to longer travel times.

This includes subway platforms, they could make a route underneath rivers or big road crossing.

ref:
valhalla/valhalla#4578
osmandapp/OsmAnd#19060
organicmaps/organicmaps#7338
graphhopper also