[REQUEST] Implement currency verification for fare (GTFS rule)

[maximearmstrong]

Is your feature request related to a problem? Please describe. Currency for a fare must be valid (existing). This is a GTFS rule implemented in Google Python validator and featured in Google Type Error as TYPE_FARE_ATTRIBUTES_INVALID_CURRENCY_TYPE.

Describe the solution you'd like Actual Google GTFS validator behaviour : verifies if currency is not missing and if it's ISO4217.

Describe alternatives you've considered

Additional context Line 43 in Error support priorities https://docs.google.com/spreadsheets/d/1vqe6wq7ctqk1EhYkgtZ0_TbcQ91vccfs2daSjn20BLE/edit#gid=0

[lionel-nj]

This error has been handled in a2b96ae0783d770fd2a1fa3da7c8cacd0461a473

[maximearmstrong]

Already supported