[#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/options.go

@@ -0,0 +1,21 @@
+package grpc
+
+import (
+	"google.golang.org/grpc"
+)
+
+type cfg struct {
+	con *grpc.ClientConn
+}
+
+func defaultCfg() *cfg {
+	return new(cfg)
+}
+
+// WithClientConnection returns option to set gRPC connection
+// to the remote server.
+func WithClientConnection(con *grpc.ClientConn) Option {
+	return func(c *cfg) {
+		c.con = con
+	}
+}