[#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/common/call.go

@@ -0,0 +1,75 @@
+package common
+
+type callType uint8
+
+const (
+	_ callType = iota
+	callUnary
+	callClientStream
+	callServerStream
+	callBidirStream
+)
+
+// CallMethodInfo is an information about the RPC.
+type CallMethodInfo struct {
+	// Name of the service.
+	Service string
+
+	// Name of the RPC.
+	Name string
+
+	t callType
+}
+
+// ServerStream checks if CallMethodInfo contains
+// information about the server-side streaming RPC.
+func (c CallMethodInfo) ServerStream() bool {
+	return c.t == callServerStream || c.t == callBidirStream
+}
+
+// ClientStream checks if CallMethodInfo contains
+// information about the client-side streaming RPC.
+func (c CallMethodInfo) ClientStream() bool {
+	return c.t == callClientStream || c.t == callBidirStream
+}
+
+func (c *CallMethodInfo) setCommon(service, name string) {
+	c.Service = service
+	c.Name = name
+}
+
+// CallMethodInfoUnary returns CallMethodInfo structure
+// initialized for the unary RPC.
+func CallMethodInfoUnary(service, name string) (info CallMethodInfo) {
+	info.setCommon(service, name)
+	info.t = callUnary
+
+	return
+}
+
+// CallMethodInfoClientStream returns CallMethodInfo structure
+// initialized for the client-side streaming RPC.
+func CallMethodInfoClientStream(service, name string) (info CallMethodInfo) {
+	info.setCommon(service, name)
+	info.t = callClientStream
+
+	return
+}
+
+// CallMethodInfoServerStream returns CallMethodInfo structure
+// initialized for the server-side streaming RPC.
+func CallMethodInfoServerStream(service, name string) (info CallMethodInfo) {
+	info.setCommon(service, name)
+	info.t = callServerStream
+
+	return
+}
+
+// CallMethodInfoBidirectionalStream returns CallMethodInfo structure
+// initialized for the bidirectional streaming RPC.
+func CallMethodInfoBidirectionalStream(service, name string) (info CallMethodInfo) {
+	info.setCommon(service, name)
+	info.t = callBidirStream
+
+	return
+}