add support for adding a custom http(s) agent

[jasisk]

Node, by default, uses a hardcoded list of CAs instead of the system CAs. In order to use a CA that is not on that list, you have to hack it into the list by patching the global https agent. Unfortunately, superagent explicitly turns off the global agent which makes patching it a no-op. So this PR adds the ability to supply the CA for superagent as an option.

[stanislav-polishchuk]

Hi @jasisk, i have some problem with your implementation of custom agent. It basically work only for first request to remote petstore. So i have implemented it in slightly different way (passing not agent instance but just options object for supergent), and it start working for me.

My question is how do you use your implementation? Because problem might be not in implementation itself, in a way that i am use it.

return new Swagger({
            url: "http://petstore.swagger.io/v2/swagger.json",
            usePromise: true,
            connectionAgent: {
                 rejectUnauthorized: false,
                 key: "/certs/example.key",
                 cert: "/certs/example.pem",
                 ca: ["/certs/example.ca.pem"]
            }})
            .then(_client => {
                //working with client
            })
            .catch(error => {
                const errMsg = `Unable to configure swagger models for ${prefix}. ${error.message}`;
                logger.error(errMsg);
            });

and the output is following:

    superagent GET https://localhost:8080/api-docs +0ms
    superagent GET https://localhost:8080/api-docs -> 200 +3s
    superagent GET https://localhost:8080/api-docs/one +12ms
    superagent GET https://localhost:8080/api-docs/two +2ms
    superagent GET https://localhost:8080/api-docs/three +1ms
    superagent GET https://localhost:8080/api-docs/one -> 403 +614ms
    superagent GET https://localhost:8080/api-docs/two -> 403 +11ms
    superagent GET https://localhost:8080/api-docs/three -> 403 +2ms