Run CI on Windows

[aantron]

Right now, it's not clear how to install this package ssl on a GitHub Actions Windows runner. This should be addressed and maintained in this repo for all downstream users. At the moment, it's not clear which version of SSL is even being linked against by the environment created by setup-ocaml in GHA. For example, windows-latest at the moment appears to be windows-2022, with software list here, which includes OpenSSL 1.1.1w. Yet trying to build ssl on it fails with the issue in https://github.com/savonet/ocaml-ssl/issues/155, so I suspect some other SSL from the setup-ocaml (Cygwin?) environment is at play. We encountered this in various forms during https://github.com/aantron/dream/pull/337, but it really should be solved upstream, perhaps by having this repo's GHA .yml file be an example.

If the CI build for this PR fails on Windows, what would it take to get it to work?

Long-term, this repo probably doesn't need a full (OS x OCaml) build matrix, but just to include: one row for testing on Windows on a certain OCaml version.