Add support for automatic request retries

[ob-stripe]

r? @brandur-stripe @richardm-stripe
cc @stripe/api-libraries

After cleaning up the code, it's now pretty straightforward to add support for automatic request retries.

This PR:

• adds a new requestWithRetries method to HttpClient
• adds a new constructor to HttpClient to accept a maxNetworkRetries value
• unfortunately, constructors are not inherited in Java, so I also had to add the same constructor to the concrete HttpURLConnectionClient class
• adds a new constructor to LiveStripeResponseGetter to allow users to provide a custom HttpClient instance

This allows users to enable automatic request retries like this:

import com.stripe.net.ApiResource;
import com.stripe.net.LiveStripeResponseGetter;
import com.stripe.net.HttpURLConnectionClient;

HttpURLConnectionClient httpClient = new HttpURLConnectionClient(2);
LiveStripeResponseGetter srg = new LiveStripeResponseGetter(httpClient);
ApiResource.setStripeResponseGetter(srg);

Retries are disabled by default to match other client libraries, but we should consider enabling them by default with a reasonable max number of attempts.

[brandur-stripe]

SUPER slick OB! Looks great. Thank you for the sleep-less test implementation as well.