Initial support for Hafas mode group configurations

This is far from complete:
- Multiple matching configurations would need multiple queries it seems,
we aren't doing that yet and just pick the first match.
- The current format only covers access/egress modes, not direct modes,
something JourneyRequest doesn't model properly yet.
- Most Hafas endpoints don't have the necessary data yet.

It's already good enough for basic combined bike/PT queries though, and
helps to test the response parsing for those scenarios.