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.

まず、pygit2 が Windows 向けにプリコンパイル済みの libgit2 をどのようにパッケージ化しているか、また pygit2.features が GIT_FEATURE_SSH をどのように報告するかを調査します。issue #902 の関連する todo 項目を確認し、Windows のビルド依存関係を検証します。Windows ディストリビューションで SSH サポートが有効になり、機能チェックが true を報告し、libssh2 の利用可能性が説明どおりに対処されていれば完了です。