Implement proto3 field presence

[Jacek Sieka]

proto3 has gained 2 significant features that are missing from the library: singular and optional fields, both relating to the presence or (lack thereof) of a field - there's an implementation guide here:

https://github.com/protocolbuffers/protobuf/blob/main/docs/implementing_proto3_presence.md

In particular, there are a few differences compared to proto2, specially in the language description (and the lack of required) - these will need consideration.