Add file appender option to flush OS buffers

Hey guys, I have already asked this question on the discussion page and it seems like that post isnt getting any reach so I am writing this issue here again, I hope that is not problem and that someone will be able to clear things out a little for me.

Since writing same thing twice doesnt make much sense I will be linking discussion below, so someone can take a look and comment if there is any idea on the problem.

issue: https://github.com/apache/logging-log4j2/discussions/2004

Thank you!