Add futures and dispatch queues

[benjamin-bader]

Along with TCntPtr, my favorite bits of MSO were the futures and the IDispatchQueue family. It would be lovely to see these here - particularly as I see they were opened for use in ReactHost elsewhere. Thanks again.

[vmoroz]

Thank you @benjamin-bader for the feature request!
Adding 'future' and 'dispatchQueue' was one of our objectives for the last week.
We currently have it copied from the private Mso fork in RNW repo (https://github.com/microsoft/react-native-windows) used by ReactHost. (The PR #45 was last change in that series.) Soon we should be able to remove the private fork from the RNW repo and start consuming it as a Nuget package.

Please note that the 'dispatchQueue' liblet implementation in this repo is different from what we have in Office. It is much more simplified. We hope that we can improve this open source version and then move it back to Office at some point.