[REQ][Protobuf-Schema] Track presence (nullable fields)

[Yurzel]

Is your feature request related to a problem? Please describe.

Generator ignores "nullable" property. Protobuf let's you send message without field set, but the other side gets default value. There is no way to say whatever the value was set (to default) or not.

Describe the solution you'd like

https://github.com/protocolbuffers/protobuf/blob/master/docs/field_presence.md

According to this link, there is a optional keyword that adds "has" and "clear" function.