Move HTTP request methods into new HttpClient class

[ob-stripe]

r? @remi-stripe
cc @stripe/api-libraries

• Move all HTTP related request stuff from LiveStripeResponseGetter into a new HttpClient class
• Rename LiveStripeResponseGetter.rawRequest to HttpClient.request
• Initialize an HttpClient instance into LiveStripeResponseGetter
• Remove LiveStripeResponseGetter.staticRequest/staticOAuthRequest. The request are now sent from the non-static request/oAuthRequest methods, which use the HttpClient instance to send the request
• Move the request duration computation into HttpClient.request

Not a breaking change, as none of the methods that are affected were public.