[#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/test/message.go

@@ -0,0 +1,68 @@
+package messagetest
+
+import (
+	"encoding/json"
+	"fmt"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/rpc/message"
+	"github.com/pkg/errors"
+	"github.com/stretchr/testify/require"
+)
+
+type jsonMessage interface {
+	json.Marshaler
+	json.Unmarshaler
+}
+
+type binaryMessage interface {
+	StableMarshal([]byte) ([]byte, error)
+	Unmarshal([]byte) error
+}
+
+func TestRPCMessage(t *testing.T, msgGens ...func(empty bool) message.Message) {
+	for _, msgGen := range msgGens {
+		msg := msgGen(false)
+
+		t.Run(fmt.Sprintf("convert_%T", msg), func(t *testing.T) {
+			msg := msgGen(false)
+
+			err := msg.FromGRPCMessage(100)
+
+			require.True(t, errors.As(err, new(message.ErrUnexpectedMessageType)))
+
+			msg2 := msgGen(true)
+
+			err = msg2.FromGRPCMessage(msg.ToGRPCMessage())
+			require.NoError(t, err)
+
+			require.Equal(t, msg, msg2)
+		})
+
+		t.Run("encoding", func(t *testing.T) {
+			if jm, ok := msg.(jsonMessage); ok {
+				t.Run(fmt.Sprintf("JSON_%T", msg), func(t *testing.T) {
+					data, err := jm.MarshalJSON()
+					require.NoError(t, err)
+
+					jm2 := msgGen(true).(jsonMessage)
+					require.NoError(t, jm2.UnmarshalJSON(data))
+
+					require.Equal(t, jm, jm2)
+				})
+			}
+
+			if bm, ok := msg.(binaryMessage); ok {
+				t.Run(fmt.Sprintf("Binary_%T", msg), func(t *testing.T) {
+					data, err := bm.StableMarshal(nil)
+					require.NoError(t, err)
+
+					bm2 := msgGen(true).(binaryMessage)
+					require.NoError(t, bm2.Unmarshal(data))
+
+					require.Equal(t, bm, bm2)
+				})
+			}
+		})
+	}
+}