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.

Inizia esaminando come pygit2 pacchettizza la propria libgit2 precompilata per Windows e come pygit2.features segnala GIT_FEATURE_SSH. Esamina l’elemento todo correlato nell’issue #902 e verifica le dipendenze di build per Windows. Il lavoro è completato quando la distribuzione Windows abilita il supporto SSH e il controllo della funzionalità restituisce true, con la disponibilità di libssh2 gestita come descritto.