Add field for distance units

### Describe the problem

There is nowhere in the current GTFS static standard specifications to list the unit used in distance measurements, causing confusion amongst users, e.g. calculating miles instead of kilometers.

### Use cases

Users would check the distance unit while ingesting a new feed, as they do for agency_timezone. Validators would check that feeds using distance measurements in shapes.txt, stop_times.txt, or fare_legs.txt have a specified unit.

### Proposed solution

An optional field in feed_info.txt or agency.txt would be a good place to house a distance_unit field, possibly with constrained options for useable unit types. With the distance requirements of fares_v2 it may eventually need to be a required field.

### Additional information

_No response_