demarcate stop signs in the graph

[fakeholz]

OSM includes information about whether there are stop signs on particular intersections. the OSM GraphBuilder currently seems to swallow this information.

[abyrd]

It does look like we check whether there are traffic lights (org.opentripplanner.openstreetmap.model.OSMNode#hasTrafficLight) but not stop signs. This should be revisited.

[github-actions]

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