Add presetupCurl callback to Gateway.

[nbelov-magdv]

I use self-signed certificate, so I need possibility to add extra options for curl.

For example, for disabling check ssl certificate I do:

const prometheus::Labels Labels = prometheus::Gateway::GetInstanceLabel("https://pushgateway.prometheus.localhost");
prometheus::Gateway Gateway{
    "https://pushgateway.prometheus.localhost", "", "sample_client", Labels, {}, {}, {},
    [&](CURL* curl)
    {
        curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0);
    }
};