[FR] Enable use of API certificate

[grahampugh]

Issue raised in MacAdmins Slack DM by Kornel:

If using a certificate for authenticating via API, so we not only need to provide API credentials in Autopkg prefs file but also point to a local .pem file.

This should be possible to add as a curl parameter, like this:

        if cert:
            curl_cmd.extend(["--cert", self.env.get("CLIENT_CERT_PATH")])