v2: Add cross-service client package

Add package with cross-service client utilities for working with transport
protocols and basic network types.

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

v2/client/proto.go

@@ -0,0 +1,23 @@
+package client
+
+import (
+	"github.com/pkg/errors"
+)
+
+type Protocol uint32
+
+const (
+	_ Protocol = iota
+	ProtoGRPC
+)
+
+var ErrProtoUnsupported = errors.New("unsupported protocol")
+
+func (p Protocol) String() string {
+	switch p {
+	case ProtoGRPC:
+		return "GRPC"
+	default:
+		return "UNKNOWN"
+	}
+}