Implement transfer distance verification

[maximearmstrong]

Is your feature request related to a problem? Please describe. A transfer distance should not be too large. This is a GTFS rule implemented in Google Python validator as ValidateTransferDistance.

Describe the solution you'd like Actual Google GTFS validator behaviour : verifies if transfer distance is not greater than 10km (error) or 2km (warning). https://github.com/google/transitfeed/blob/d727e97cb66ac2ca2d699a382ea1d449ee26c2a1/transitfeed/transfer.py#L134

Describe alternatives you've considered

Additional context Line 192 in Error support priorities https://docs.google.com/spreadsheets/d/1vqe6wq7ctqk1EhYkgtZ0_TbcQ91vccfs2daSjn20BLE/edit#gid=0

[barbeau]

cc @timMillet for transfer distance threshold best practices in validator

[isabelle-dr]

We usually don't add rules that are not clearly specified in the spec or best practices (this is the case here), but we make exceptions for what the community thinks is valuable to check (point_near_origin, fast_travel_between_consecutive_stops, unused_trip, just to name a few). This is done on a case-by-case basis, by discussing with the community. We are in favor of having the spec modified first before adding the check in the validator.