[#263] Define generic protobuf message

Define `Message` interface of the generic protobuf message. In the initial
implementation, the message should be convertible to and from related gRPC
message.

Implement encoding functions over the `Message` that can:

 * unmarshal the `Message` via related gRPC message;
 * decode the `Message` from JSON format via related gRPC message;
 * encode the `Message` to JSON.

Implement nested `test` package for testing the implementation.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

rpc/message/encoding.go

@@ -0,0 +1,48 @@
+package message
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/rpc/grpc"
+	"google.golang.org/protobuf/encoding/protojson"
+	"google.golang.org/protobuf/proto"
+)
+
+// GRPCConvertedMessage is an interface
+// of the gRPC message that is used
+// for Message encoding/decoding.
+type GRPCConvertedMessage interface {
+	grpc.Message
+	proto.Message
+}
+
+// Unmarshal decodes m from its Protobuf binary representation
+// via related gRPC message.
+//
+// gm should be tof the same type as the m.ToGRPCMessage() return.
+func Unmarshal(m Message, data []byte, gm GRPCConvertedMessage) error {
+	if err := proto.Unmarshal(data, gm); err != nil {
+		return err
+	}
+
+	return m.FromGRPCMessage(gm)
+}
+
+// MarshalJSON encodes m to Protobuf JSON representation.
+func MarshalJSON(m Message) ([]byte, error) {
+	return protojson.MarshalOptions{
+		EmitUnpopulated: true,
+	}.Marshal(
+		m.ToGRPCMessage().(proto.Message),
+	)
+}
+
+// UnmarshalJSON decodes m from its Protobuf JSON representation
+// via related gRPC message.
+//
+// gm should be tof the same type as the m.ToGRPCMessage() return.
+func UnmarshalJSON(m Message, data []byte, gm GRPCConvertedMessage) error {
+	if err := protojson.Unmarshal(data, gm); err != nil {
+		return err
+	}
+
+	return m.FromGRPCMessage(gm)
+}