Add support for SearchResult objects.

[dcr-stripe]

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Summary

Adds support for search_result objects returned by the Stripe API, including adding a templated SearchResult class and a Search trait which can be added to resources to support invoking search APIs.

This behaves similarly to Collection in that they both wrap around a list of objects and provide pagination. However the SearchResult object paginates by relying on a next_page token included in the response rather than using object IDs + starting_before/ending_after. Thus, only forward pagination is supported

This was revived from #1134 with the following API changes:

• page is used in the request
• next_page is always returned, just null when there's no more page.

Test plan

Added unit tests. We'll want to add some more rigorous tests once stripe-mock has been updated to include at least one API that returns this resource.

Post-merge

The generation for init.php and lib/Util/ObjectTypes.php should be updated to include these new classes going forward.

[dcr-stripe]

r? @yejia-stripe