`cctz::TimeZoneIf` implementation that is compatible with Windows time APIs

Inspired by the following message in `README.md`, I actually created a full implementation of `cctz::TimeZoneIf` by using Windows time APIs (actually their source data stored in the Windows registry).

> They will also work on **Windows** if you install the zoneinfo files. We are interested, though, in an implementation of the cctz::TimeZoneIf interface that calls the Windows time APIs instead. Please contact us if you're interested in contributing.

Here is my draft pull request.

* https://github.com/google/cctz/pull/327

If the team is still interested in such an implementation, I'm planning to split the above PR into 2 or 3 smaller ones.