Allow to override default ssh transport_cb - in order to allow third party ssh transports

*new*:
W/o this patch it is not possible to have a third party ssh transport_cb if GIT_SSH is disabled.

*old*:
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.
You're leaking the transport. Since you're not going to actually use it for anything, I'd recommend returning the magic value directly instead of allocating some memory that's not going to be doing anything.