Allow to override default ssh transport_cb

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.
To be honest: This isn't necessary. If someone compiled libgit2 w/o GIT_SSH and tries to use ssh:// the error "Unsupported URL protocol" is sufficient.