Allow add log with custom timestamp

[nmduong]

I need to add a log with a custom timestamp because I need to write a log from native (Plugin) to FLogs while the application is running.
But if I use MethodChannel to call the function from native to flutter. The timestamp would be incorrect so my log is difficult to read.
So I think if I get the timestamp from native and send it to Flutter. That can help resolve my issue.

Thanks,