JSON support for changeset upload

[mmd-osm]

Back in https://github.com/openstreetmap/openstreetmap-website/issues/3501 we've talked about adding JSON support for a few remaining endpoints. Unfortunately, this project never took off. Why not give it another try now.

I found a streaming json parser based on yajl called sjparser, that has schema support. It offers a much saner interface than yajl alone, and emits one OSM element at a time. It also hides all that ugly state handling, unlike the XML parser we have in place today.

• Define new format specifications for JSON based messages:
  • /api/0.6/changeset/nnn/upload
    • Input: OSMChange, based on OSM JSON format -> some examples in test_parse_osmchange_json_input.cpp
    • Output: diffResult: TODO

There are a few other changeset related endpoints (create / update) that don't support JSON. This can be addressed in another PR. Single object edit operations will continue be out of scope for this repo.

[mmd-osm]

Created some issues for sjparser:

• https://gitlab.com/dhurum/sjparser/-/issues/1
• https://gitlab.com/dhurum/sjparser/-/issues/2

[mmd-osm]