Starting on windows

[aloysbaillet]

Added windows Dockerfile which can already build a few packages, I've tested ninja and cmake for now.

N.B. that building clang from source makes little sense as binaries are already available for windows. Building python from source also makes little sense... We need to think about which packages we really want to handle properly in Conan.

[aloysbaillet]

Thanks JF! Sorry I was meant to set this PR as a draft for now, still quite WIP :)

I think using a windows container has at least these 2 nice advantages:

• more reproducibility especially with things like python installs which can only be done once per version, with secret registry keys that could change build behaviours etc... This is quite key for building things locally and having some confidence that these will build as well in the GitHub actions.
• more symmetric code with the linux builds, which means simpler code overall

That said, once we have the packages built and uploaded to Conan, I don't see much value in using containers to download and use the packages, I don't expect client repos to use docker for windows builds. Hopefully we can provide a GitHub action that installs Conan with the right settings on the windows GHA VMs when running windows CI.

Happy to discuss at the next CI meeting.