[#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/message.go

@@ -0,0 +1,43 @@
+package message
+
+import (
+	"fmt"
+
+	"github.com/nspcc-dev/neofs-api-go/rpc/grpc"
+)
+
+// Message represents raw Protobuf message
+// that can be transmitted via several
+// transport protocols.
+type Message interface {
+	// Must return gRPC message that can
+	// be used for gRPC protocol transmission.
+	ToGRPCMessage() grpc.Message
+
+	// Must restore the message from related
+	// gRPC message.
+	//
+	// If gRPC message is not a related one,
+	// ErrUnexpectedMessageType can be returned
+	// to indicate this.
+	FromGRPCMessage(grpc.Message) error
+}
+
+// ErrUnexpectedMessageType is an error that
+// is used to indicate message mismatch.
+type ErrUnexpectedMessageType struct {
+	exp, act interface{}
+}
+
+// NewUnexpectedMessageType initializes an error about message mismatch
+// between act and exp.
+func NewUnexpectedMessageType(act, exp interface{}) ErrUnexpectedMessageType {
+	return ErrUnexpectedMessageType{
+		exp: exp,
+		act: act,
+	}
+}
+
+func (e ErrUnexpectedMessageType) Error() string {
+	return fmt.Sprintf("unexpected message type %T: expected %T", e.act, e.exp)
+}