Research API designs for Async File IO

Investigate an API pattern which enables us to reflect asynchronous operations which fit well into the C++20 world (e.g. co_await) and at the same time provide a solution for C++17 too (until Qt can make the change).

Ultimately we want to use this together with https://bugreports.qt.io/browse/QTBUG-129365 for asynchronous resource access (e.g. QFileV2,QAysncFile or QAsyncResource).

This is generic API pattern research and could apply to other domains besides Qt's IO domain.