Add fare_product_type field to fare_products.txt

### Describe the problem

The distinction between the price of a regular fare and a transfer is common among passengers, yet there is no way in the Fares v2 specification to distinguish a regular fare from a transfer. The fare_transfer_rules.txt can associate a fare product with transfer rules but cannot indicate a fare product is for a transfer only. fare_transfer_rules.txt is also optional and may not always be present to discern whether a fare product includes a transfer.

### Use cases

Passengers viewing fare product lists can discern whether a fare product convers a trip only, a trip with transfers, or the cost of a transfer only.

### Proposed solution

Allow publishers to distinguish between regular fare products and transfer fares.

Optional `fare_product_type` as an enum would resolve the issue and allow consumers to clearly present whether a fare product purchases a regular trip, a regular trip with a transfer, or a transfer only.

'0' or empty - Regular. The product is for the purchase of a regular trip and transfer information may be specified in fare_transfer_rules.txt.
'1' - Transfer Only. The product is for the purchase of transfer only and transfer information may be specified in fare_transfer_rules.txt.

### Additional information

The Fares v2 specification does not currently make it possible to distinguish whether a fare product includes the cost of a trip, a trip with transfers, or a transfer only.