Investigate usage of alternative ways to process PBFs

Since `imposm` is deprecated since a long while and we really don't wanna start maintaining that, we need to investigate alternative ways to process PBFs:

- `pyosmium` could be an OK candidate. Very limited API, but does accept callbacks for OSM types
- `https://pypi.org/project/esy-osm-pbf/` seems to be a relatively new package, doing what we'd need (couldn't find in on any VCS platform though..)
- use `osmium`/`osmosis` or other command-line utilities or even Pelias' [pbf2json utitlity](https://github.com/pelias/pbf2json). All at the expense of creating more non-Python dependencies..

So, these will have to be evaluated a little in terms of performance with clear favorites being the first two options, as only `protobuf` lib as non-Python dep needed.