`PedestrianCost` does not prefer sidewalks by default

I recently mapped a sidewalk separately from the carriageway that was previously just tagged on the carriageway, but noticed that once those changes trickled down into the FOSSGIS instance, Valhalla's routing did not seem to pick them up, while OSRM and GraphHopper do.

https://www.openstreetmap.org/directions?engine=fossgis_valhalla_foot&route=51.797253%2C8.552288%3B51.799858%2C8.548366#map=18/51.798561/8.550335

Turns out that by default `sidewalk_factor` and `walkway_factor` are 1, and that we don't penalize any road by use or classification in any way, so naturally it takes the shortest path, which is along the main carriage way.

Start by locating the PedestrianCost implementation and the sidewalk_factor and walkway_factor defaults described in the issue. Trace how pedestrian road use or classification affects route costing, then verify that a mapped sidewalk is preferred over the main carriageway while preserving sensible pedestrian routing behavior.

Written by the indexing model from the issue text.