HTTP retry method can accept array as first param (#566)
* HTTP retry method can accept array as first param
* Update type hints in PendingRequest class
* Add backoff property to PendingRequest class
* Add retry method to PendingRequest class
* Fix mergeableOptions typo in PendingRequest class
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Co-authored-by: Deeka Wong <8337659+huangdijia@users.noreply.github.com>