fahrplan-updater, gcs: add new crates to store fahrplan updates in GCS

- fahrplan currently updates schedules with another thread that does all
  the dirty work. However, this is suboptimal for a variety of reasons -
  for example, it requires fahrplan to store state, which we'd like to
  avoid.
- The new fahrplan-updater crate copies all of the fahrplan update code,
  and allows doing updates based on a tspl-fahrplan sqlite database
  stored in Google Cloud Storage.
- Furthermore, it will automatically reinitialize the database if the
  GCS copy either doesn't exist, or is broken or corrupted (e.g. it
  missed an update file). This is an improvement over the status quo.
- In the future, fahrplan will download this storage object at boot,
  meaning we'd be able to make it a Deployment instead of a StatefulSet,
  and have multiple instances.