Improve compatibility to GTFS and NetEx

[opyh]

Both formats have interesting attributes that A11yJSON should be more compatible with:

https://github.com/NeTEx-CEN/NeTEx/blob/17c3b065ed5772e751dcdc6477360629604d2a0a/xsd/netex_part_1/part1_ifopt/netex_ifopt_equipmentAccess_version.xsd#L455

https://developers.google.com/transit/gtfs/reference/#pathwaystxt