[#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/client.go

@@ -0,0 +1,23 @@
+package grpc
+
+// Client represents client for exchanging messages
+// with a remote server using gRPC protocol.
+type Client struct {
+	*cfg
+}
+
+// Option is a Client's constructor option.
+type Option func(*cfg)
+
+// New creates, configures via options and returns new Client instance.
+func New(opts ...Option) *Client {
+	c := defaultCfg()
+
+	for _, opt := range opts {
+		opt(c)
+	}
+
+	return &Client{
+		cfg: c,
+	}
+}