Change "in_reply_to_discussion_id" to integer

From @ekudel at #410504 (comment 1501124592)+

Hi guys. Thank you for implementing the API! We still have one issue with it though. You've marked "in_reply_to_discussion_id" parameter for "Create Draft Note" endpoint as Integer, but merge request discussion's ID is String. So we cannot create a reply draft note with the API. Or do we miss anything?