Add support for ssh tunnels (like plink) on win32

This adds support for SSH tunnel helpers (like plink) on Win32.

In the current form it's quite hardcoded, because there is no proper fallback for third party ssh transports in https://github.com/csware/libgit2/blob/118d1f9e3efe6308d134c8fef1b55a871db1a58a/src/transport.c#L107.