Extend ABI to expose journeys, footpaths, cancellations

[traines-source]

This finally addresses most of the open todos from #66, among other things:

• handle RT track changes, in_allowed/out_allowed
• expose footpaths
• expose querying/journeys

There is one change I made that has an impact outside of the ABI: For absolute-time-based GTFSRTs (ev.has_time()), the time-travel-corrected time will be propagated in L119 just like for delay-based GTFSRTs in L94. I think this was a bug.

[felixguendling]

Regarding formatting: We switched to Clang-18 (clang-format-18).

[felixguendling]

@mority can you please check the GTFS RT part with the proposed propagation bugfix? Thank you :)

[traines-source]

Regarding formatting: I just forgot to format this file, and the .clang-format config, which my editor should use, seems to be fine even if it has not been updated in the past two years?

More details regarding propagation: As far as I understood this was meant to avoid vehicles travelling back in time due to missing or wrong realtime updates, e.g. having a delay of one hour at one stop and arriving on time at the next stop which is only a minute away. I think I even encountered this in the wild back in the day (with the RT feed from gtfs.de which uses absolute timestamps), but I'm not sure anymore, would have to check.

[traines-source]

Sorry for the wait, good points. I have refactored accordingly, but instead of an enum I have used std::optional on the C++ side and an additional bool on the C side, because the delay actually also needs to be ignored/undefined for events where stop metadata (track or in/out_allowed) is changed.