Summary:
In PosixFileSystem, IO uring support is opt-in. If the support is not enabled by the user, then ignore the async_io ReadOption in MultiGet and iteration at the top, rather than follow the async_io codepath and transparently switch to sync IO at the FileSystem layer.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11296

Test Plan: Add new unit tests

Reviewed By: akankshamahajan15

Differential Revision: D44045776

Pulled By: anand1976

fbshipit-source-id: a0881bf763ca2fde50b84063d0068bb521edd8b9