Write parser OSC > FGB

Write now we parse OSC files into json using `osm-adiff-parser`, then geojson using `real-changesets-parser` and then roll up the files into one geojson and use Ogr2Ogr to create an FGB for each time resolution.

To avoid the intermediary steps, it would be interesting to build a Rust parser that reads in OSC XML and automatically packs features into FGBs. We could use [FGBwriter](https://docs.rs/flatgeobuf/latest/flatgeobuf/struct.FgbWriter.html#method.add_feature_geom) and Geozero on the write side, but we would still need to implement `adiff-parser` using a custom XML to Feature parser.