(Beta) simplify stripe.Backend interface

[richardm-stripe]

Along similar lines to stripe/stripe-java#1702 in stripe-java.

Summary

Introduces stripe.StripeRequest struct, and changes Backend interface:

 type Backend interface {
- 	Call(method, path, key string, params ParamsContainer, v LastResponseSetter) error
+  	Call(req StripeRequest, v LastResponseSetter) error
- 	CallStreaming(method, path, key string, params ParamsContainer, v StreamingLastResponseSetter) error
+ 	CallStreaming(req StripeRequest, v StreamingLastResponseSetter) error
- 	CallRaw(method, path, key string, body *form.Values, params *Params, v LastResponseSetter) error
- 	CallMultipart(method, path, key, boundary string, body *bytes.Buffer, params *Params, v LastResponseSetter) error
- 	SetMaxNetworkRetries(maxNetworkRetries int64)
 }

Setting up StripeRequest is broken up into two steps: To achieve the equivalent of b.Call(method, path, key, params, &result), you now do

sr := stripe.NewStripeRequest(method, path, key)
sr.SetParams(params)
b.Call(sr, &result)

To achieve the equivalent of b.CallRaw(method, path, key, body, params, &result):

sr := stripe.NewStripeRequest(method, path, key)
sr.SetRawForm(params, body)
b.Call(sr, &result)

To achieve the equivalent of b.CallMultipart(method, path, key, boundary, body, params, &result):

sr := stripe.NewStripeRequest(method, path, key)
sr.SetMultipart(params, boundary, body)
b.Call(sr, &result)

Why

This is breaking, but this:

• will allow us to pass new, optional data to.Call by adding it onto StripeRequest. Otherwise, we would have to break the interface and add a new parameter to .Call or a new method to Backend, or we would have to smuggle the data onto Params, which isn't really an appropriate place.
  • For instance: usage that we are soon adding to all libraries,
  • or ApiMode that is used by RawRequest to send preview requests.
• it decreases the surface area necessary to implement Backend, which should make the experience better for future users who need to write an implementation of Backend (for mocking in their tests?) -- Call is essentially a convenience wrapper for CallRaw, but by putting both on the interface, we force users to implement both.

Planned follow-ups

• I should be able to remove RawRequestBackend now and subsume it into Backend. (Will do in a follow-up, to keep the diff manageable).
• Usage

[richardm-stripe]

Closing: we're going to resurrect this when there are other reasons to make infrastructure changes. For now, we're going to avoid the breaking change and smuggle any additional parameters onto Params.