Provide an annotation driven way of declaring protobuf metadata

The main goal of this is to simply marshalling of java objects to protobuf using the Protostream library.

Instead of providing a Protostream MessageMarshaller implementation and a proto schema file it would be nice to have an alternative way of just adding minimal annotations to a Java class (and its fields) to assist on the fly generation (via reflection) of the proto schema. The Protostream library should also infer the marshaller so it would not require a manually implemented one. The annotation should require minimal info (tag number) and the rest should be inferred based on common sense defaults (protobuf type, java collection type, collection element type ...) but should be possible to override. The auto-generated schema should be available to users and they should be able then to use it as reference if they need it to implement domain model classes and marshallers for other languages.

The user should still be able to specify a manually created schema (besides the annotated classes) and in that case the Protostream library should check the auto-inferred schema against the provided one and ensure they match.