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.
Your suggestion is to set *transport to the magic number and return 0? That doesn't work: giterr__check_version tries to dereference the pointer in order to check its versionnumber.