Add native Windows/VS2019 CI
This MR adds support for building Mesa natively under Windows/VS2019. Like the Linux build, this uses a two-stage base container build followed by a Mesa-only build. Unlike the Linux build, it cannot reuse ci-templates because Windows does not support Docker-in-Docker, nor Podman or Skopeo. The same is instead open-coded with PowerShell.
/cc @brianp @krh @jrfonseca