relax the requirement for in-seat transfer to allow attach / detaching vehicles in the middle of a trip

### Describe the problem

Currently, the in-seat transfer says that:

> The last stop of from_trip_id SHOULD be geographically close to the first stop of to_trip_id, and the last arrival time of from_trip_id SHOULD be prior but close to the first departure time of to_trip_id. The last arrival time of from_trip_id MAY be later than the first departure time of to_trip_id in case the to_trip_id trip is occurring the subsequent service day.

However, this modelling does not allow the specification where carriages are attached / detached in the middle of a trip, which is common in the National Rail network.

### Use cases

I would like to model the joining / splitting of train services, where carriages are attached / detached not at the end / beginning of a trip, but instead, in the middle of a trip.

For example, a train trip runs from London Bridge to Caterham, and another trip runs from Purley to Tattenham Corner by detaching a portion from the London Bridge to Caterham trip.

### Proposed solution

Relax the requirement of `transfer_type = 4` to allow the transfer taking place between two trips, where the transfer stop is a terminus (start stop / end stop) for one of the trip, and any (including start / end / intermediate) stop for the other.

### Additional information

_No response_