Add validation for transfers.txt transfer_type=4 and 5

[bdferris-v2]

Describe the problem

As discussed in google/transit/pull/303, the spec was recently amended to support two new values for transfers.txt transfer_type, 4 and 5, in support of in-seat transfers. We should extend the validator to accept these new values.

Describe the new validation rule

Extend GtfsTransferTypeEnum

Sample GTFS datasets

No response

Severity

No response

Additional context

No response