transfers.txt: Add optional field service_id

### Describe the problem

I suggest adding the optional field service_id to transfers.txt so that secured connections can be restricted to the days on which they are actually secured.

### Use cases

Our network has guaranteed connections (transfer_type = 1 with from_trip_id and to_trip_id) between journeys, but not on all days on which these journeys are operated. On days without guaranteed connections, there may be a through connection (block transfer), a guaranteed connection at another stop or – for operational reasons – no guaranteed connection at all.

### Proposed solution

Add the optional field service_id to transfers.txt

### Additional information

_No response_