Write to JSON using rapidjson writer instead of hardcoded strings

[vasdommes]

In our code, we write JSON manually - this is hard to read and edit. We should use rapidjson::Writer instead of reinventing the wheel.

Example of our code: https://github.com/davidsd/sdpb/blob/3169d9f80bec1399572dfa10fdb1b72d34a40c9a/src/pmp2sdp/write_block_data.cxx#L14-L37