[GTFS-Fares v2] Non-sequential Legs Transfer

### Problem to solve
The [Transfer Semantics document](https://docs.google.com/document/d/1Lqh3eI2YoxAaLSqUePCkcQ7WmyD-d-eXeDYymPnUfLQ/edit#heading=h.l3vb2g3af1fg) mentions that there are systems where transfers are time-based, such that riders can acquire fare discounts for non-direct transfers. For example, in a journey with four legs (Leg1 → Leg2 → Leg3 → Leg4), where Leg1 and Leg4 belong to SystemA, and Leg2 and Leg3 belong to SystemB. In SystemA, even if riders travel through a different system(B) during the journey, Leg4 can still acquire a discount from Leg1, as long as it remains within a specific amount of time. However, the current Fares v2 spec seems unable to differentiate whether non-direct transfers are allowed.

Furthermore, the current spec also appears unable to distinguish cases where the `transfer_count`and the timer for `duration_limit` should be "reset". For instance, in the example above, when transferring to Leg2, it might be necessary to "reset" the timer and `transfer_count` for SystemB.

We'd love to see any potential solutions and real-world use cases. Please feel free to share any thoughts here.