add file_copy example

[kirkshoop]

requires #347 and #348

This is the first working version. Cleanup and improvements needed. It implements a throttling algo adhoc. :( It runs a bit slower than the std::filesystem version - I need someone with more linux knowledge to point me in the right direction. I speculate that std::filesystem does the entire copy in kernel by using a file copy syscall per file. this is using liburing to actually open and read and write 64000 byte blocks from user mode.