Add Windows support

This is a know issue, and one we would like to fix, but until our dependencies support Windows there is little we can do. Avoiding these dependencies is also hard (e.g. grpc-swift-2 indirectly depends on swift-nio-extras):

* https://github.com/apple/swift-nio-extras/issues/325
* https://github.com/swift-server/async-http-client/issues/926

Start by reviewing the dependency blockers named in the issue: grpc-swift-2, swift-nio-extras, and async-http-client, along with the linked dependency issues. The work is complete when those dependencies support Windows and google-cloud-swift can support Windows without the stated compatibility blockers.

Written by the indexing model from the issue text.