a smart transport
the certificate to pass to the caller
whether we believe the certificate is valid
the hostname we connected to
the return value of the callback: 0 for no error, git_error_code.GIT_PASSTHROUGH to indicate that there is no callback registered (or the callback refused to validate the certificate and callers should behave as if no callback was set), or < 0 for an error
Call the certificate check for this transport.