Allow credential and certificate checking callbacks to return
GIT_PASSTHROUGH, indicating that they do not want to act.
Introduce this to support in both the http and ssh callbacks.
Additionally, enable the same mechanism for certificate validation.

This is most useful to disambiguate any meaning in the publicly exposed
credential and certificate functions (`git_transport_smart_credentials`
and `git_transport_smart_certificate_check`) but it may be more
generally useful for callers to be able to defer back to libgit2.