rpcserver+cmd/lncli: implement DecodePayReq

This commit implements the newly added RPC to decode payment requests
passed over the command line or directly via gRPC.

With this tool, users can now examine payment requests they see in the
wild for diagnostic or debugging purposes.

cmd/lncli/main.go

@@ -71,6 +71,7 @@ func main() {
 		QueryRouteCommand,
 		GetNetworkInfoCommand,
 		DebugLevel,
+		DecodePayReq,
 	}
 
 	if err := app.Run(os.Args); err != nil {