[FEATURE] AVRO serialization support

[mgovers]

Currently, PGM supports serialization to JSON and MessagePack, according to its own JSON serialization schema. The schemas already are relatively closely compatible with AVRO schemas. Since AVRO is becoming the de-facto standard for binary serialization, outclassing MessagePack in supporty by users, it would be good to add this serialization format as well.

Just like for nlohmann_json for JSON and msgpack-cpp for MessagePack, there an Avro C++ library (https://avro.apache.org/docs/1.11.0/api/cpp/html/index.html) to help with AVRO serialization.