Add streaming requests

[richardm-stripe]

Notify

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

Description

Implements support for streaming requests.

How this will be used

// In foo/client.go, something like:
func (c Client) PDF(id string, params *stripe.FooPDFParams) (*stripe.APIStream, error) {
	path := stripe.FormatURLPath("/v1/foo/%s/pdf", id)
	stream := &stripe.APIStream{}
	err := c.B.CallStreaming(http.MethodGet, path, c.Key, params, stream)
	return stream, err
}

and then the user can

stream, err := stripe.foo.PDF("foo_xyz", nil)
if err != nil {
  return err  
}
writer, err := os.Create("/tmp/foo.pdf")
defer writer.Close()
io.Copy(writer, stream.LastResponse.Body)

It might help to review this commit-by-commit. There's some refactoring into helper functions.

[dcr-stripe]

Looks good overall! Mainly minor comments - thanks Richard!

[richardm-stripe]

r? @dcr-stripe thanks for the great review! Went a little further in the refactoring/reducing duplication and addressed all comments.

[dcr-stripe]

Looks great - thanks Richard! 🥳 Just one comment about naming. Thanks for all the work on this!!