Output predictions as enhanced GTFS realtime

[lenaschimmel]

We already prepared the possibility to make our predictions available via GTFS realtime. What's left to do:

• [ ] Get an extension ID from Google / the GTFS group
• [ ] Finalize the way we encode the curves
• [ ] Optional: Think of parameters that can be provided via the URL, so that the stream consumer can select / filter the output
• [ ] Implement an endpoint on our server (monitor) or create a new server process/container for this
• [ ] Publish the stream URL