Change StripeResponseGetter to take a single APIRequest object

[richardm-stripe]

Background/Approach

We keep wanting to add more arguments to
StripeResponseGetter.request and StripeResponseGetter.requestStream. We added ApiMode when we introduced StripeClient, and now we want to introduce "usage" (pending in #1698)

Rather than keep accumulating overloads and defining default implementations, we can just define a single class to represent the arguments that .request accepts. We'll still have to add new constructors (or setters or whatnot) when we add to this object, so it doesn't make the problem completely go away, but it's generally easier to backwards-compatibly extend an object with new constructors and new fields than it is to extend a method on an interface.

Summary

• This PR introduces a new class APIRequest (please bikeshed this name), and a new overload to StripeResponseGetter.request and StripeResponseGetter.requestStream that accepts APIRequest instead of all the arguments it currently accepts.
• This PR also changes all internal usages to use APIRequest.
• The new overload has a default implementation, so this change can be non-breaking.
• I did not add the @deprecated annotation to the old overload. But I can.

[pakrym-stripe]

Looks good but a few questions. The biggest one around hierarchy with RawApiRequest

[richardm-stripe]

re-requesting review, would just like a stamp on the naming of
BaseApiRequest

[anniel-stripe]

BaseApiRequest looks good to me!