Support calling the internal API over HTTPS

We need to support calling the internal API over HTTPS.

We need to allow disabling certificate validation: https://gitlab.com/gitlab-org/gitlab-shell/blob/6109b64ed81f80e90bf094ad1f1ded8f5e5f8324/lib/http_helper.rb#L35

We currently also support specifying a custom certificate file or path: https://gitlab.com/gitlab-org/gitlab-shell/blob/6109b64ed81f80e90bf094ad1f1ded8f5e5f8324/lib/http_helper.rb#L103-116