toProto3Json incompatible with dart:convert

According to jsonEncode from dart:convert:
`If toEncodable is omitted, it defaults to a function that returns the result of calling .toJson() on the unencodable object.`

In order to serialize an object tree, all nested objects should have the method .toJson(). In order to support serialization of objects that contain protobuf messages, toProto3Json could be renamed to toJson.