Enhancement: Use /api/v1/trains/checkin with transitous stop ids

[networkException]

Describe the problem

When implementing support for checking in using the api it is not possible to use transitous stop ids, as the start and destination station fields need to be a träwelling internal id or an ibnr.

While this could be solved by adding an endpoint to query träwelling's data model for transitous station identifiers, this only works with träwelling having fetched the stations previously. Ideally the checkin api would fetch the transitous trip first, discovering all stopover stations in the process.

[HerrLevin]

I've removed this from the original migration parent issue because this is not directly needed to fulfil the direct functionality.

While we could accept the motis/transitous endpoint ID, we've decided in the past that we don't want to work with external identifiers in our api. In the long run we wanted to possibly integrate multiple API sources, which would only make this more difficult. The IBNR is a remaining legacy of old processes and internal workings of Träwelling and the (slow) refactoring of the ui.

As a workaroud you could still query the autocomplete endpoint of Träwelling with the exact name of your station. That way you'll directly get a Träwelling ID.

We could still implement this but with the following suggestion:

• (maybe) no extra endpoint. Motis has no direct endpoint for fetching station data. We'll always need to fetch the motis departure endpoint and iterate over every single departure for fetching the data

We'll look into it, how/what we're going to do with it, once we're over the transitous migration.