Add infrastructure for test-helper methods

[pakrym-stripe]

When generating test_helper methods on a resource we group them into a TestHelpers nested class:

        @test_helpers
        @custom_method("pdf", http_verb="get")
        class Foo(CreateableAPIResource):
          OBJECT_NAME = "foo"
          def pdf(self, idempotency_key=None, **params):
               #...

          @custom_method("boop", http_verb="post")
          class TestHelpers(APIResourceTestHelpers):
            def boop(self, idempotency_key=None, **params):
              url = self.instance_url() + "/boop"
              headers = util.populate_headers(idempotency_key)
              self.resource.refresh_from(self.resource.request("post", url, params, headers))
              return self.resource

We need a few changes to infrastructure to get this working:

1. APIResourceTestHelpers that contains class_url and instance_url generation logic (so test_helpers is included in URLs).
2. test_helpers decorator to add test_helpers property to the resource class.
3. custom_method logic adjusted to support having static method on a nested class.

[pakrym-stripe]

re-r? @richardm-stripe

[richardm-stripe]

This looks good - let's wait to merge until the first test_helper method is released.

[dcr-stripe]

Thanks Pavel! Mainly all minor comments about adding documentation, otherwise this LGTM!

PTAL @pakrym-stripe

[pakrym-stripe]

PTAL @dcr-stripe ?

[dcr-stripe]

LGTM, this looks great! Thanks Pavel!