Add SSH support on Windows platforms

So pygit2 now comes with a pre-compiled libgit2. That's nice.
But on Windows platforms it seems that SSH support is not enabled in that pre-compiled libgit2 version. Even if one installs libssh2, the included libgit2 will not work with SSH.

E.g.:

bool(pygit2.features & pygit2.GIT_FEATURE_SSH)

Will still report false. Not sure if I'm missing something here but if I understand this correctly then it would be nice if the included libgit2 would enable SSH support and eventually even come with a pre-compiled libssh2 for Windows platforms.

I saw that this was on the todo list here: https://github.com/libgit2/pygit2/issues/902
But it's not ticked and I'm not sure it was ever done.

Commencez par examiner comment pygit2 empaquette sa libgit2 précompilée pour Windows et comment pygit2.features signale GIT_FEATURE_SSH. Examinez l’élément todo associé dans l’issue #902 et vérifiez les dépendances de build de Windows. Le travail est considéré comme terminé lorsque la distribution Windows active la prise en charge de SSH et que la vérification de la fonctionnalité renvoie true, la disponibilité de libssh2 étant traitée comme indiqué.