[#263] Implement client for exchanging raw messages using gRPC protocol

Implement gRPC client that can uniformly execute any RPC on the remote
server. In the primary implementation, the client is a thin wrapper over
gRPC client connection that is required to create the client. In the future,
it is planned to expand the library with convenient functionality.

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

rpc/grpc/init.go

@@ -0,0 +1,65 @@
+package grpc
+
+import (
+	"io"
+
+	"github.com/nspcc-dev/neofs-api-go/rpc/common"
+	"google.golang.org/grpc"
+)
+
+// Message represents raw gRPC message.
+type Message interface{}
+
+// MessageReadWriter is a component interface
+// for transmitting raw messages over gRPC protocol.
+type MessageReadWriter interface {
+	// ReadMessage reads the next message from the remote server,
+	// and writes it to the argument.
+	ReadMessage(Message) error
+
+	// WriteMessage sends message from argument to remote server.
+	WriteMessage(Message) error
+
+	// Closes the communication session with the remote server.
+	//
+	// All calls to send/receive messages must be done before closing.
+	io.Closer
+}
+
+type streamWrapper struct {
+	grpc.ClientStream
+}
+
+func (w streamWrapper) ReadMessage(m Message) error {
+	return w.ClientStream.RecvMsg(m)
+}
+
+func (w streamWrapper) WriteMessage(m Message) error {
+	return w.ClientStream.SendMsg(m)
+}
+
+func (w *streamWrapper) Close() error {
+	return w.ClientStream.CloseSend()
+}
+
+// Init initiates a messaging session within the RPC configured by options.
+func (c *Client) Init(info common.CallMethodInfo, opts ...CallOption) (MessageReadWriter, error) {
+	prm := defaultCallParameters()
+
+	for _, opt := range opts {
+		opt(prm)
+	}
+
+	stream, err := c.con.NewStream(prm.ctx, &grpc.StreamDesc{
+		StreamName:    info.Name,
+		ServerStreams: info.ServerStream(),
+		ClientStreams: info.ClientStream(),
+	}, toMethodName(info))
+	if err != nil {
+		return nil, err
+	}
+
+	return &streamWrapper{
+		ClientStream: stream,
+	}, nil
+}