[#263] Implement client for exchanging raw Protobuf messages

Implement generic `Client` that can communicate with the remote server via
protobuf `Message`'s. The client can uniformly execute any protobuf RPC
on the remote server using any of the supported transport protocols.
Currently only gRPC protocol is supported.

Additionally implement helpful functions to transmit messages by one of the
flow types: unary, client- or server-side stream.

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

rpc/client/connect.go

@@ -0,0 +1,41 @@
+package client
+
+import (
+	"context"
+	"errors"
+
+	"github.com/nspcc-dev/neofs-api-go/rpc/grpc"
+	grpcstd "google.golang.org/grpc"
+)
+
+func (c *Client) createGRPCClient() (err error) {
+	c.gRPCClientOnce.Do(func() {
+		if err = c.openGRPCConn(); err != nil {
+			return
+		}
+
+		c.gRPCClient = grpc.New(grpc.WithClientConnection(c.conn))
+	})
+
+	return
+}
+
+var errInvalidEndpoint = errors.New("invalid endpoint options")
+
+func (c *Client) openGRPCConn() error {
+	if c.conn != nil {
+		return nil
+	}
+
+	if c.addr == "" {
+		return errInvalidEndpoint
+	}
+
+	var err error
+
+	dialCtx, cancel := context.WithTimeout(context.Background(), c.dialTimeout)
+	c.conn, err = grpcstd.DialContext(dialCtx, c.addr, grpcstd.WithInsecure())
+	cancel()
+
+	return err
+}