Use MR draft and deprecate work_in_progress

Hey,

I notice that the current MR schema only has the field work_in_progress, which has been officially deprecated: https://docs.gitlab.com/ee/api/merge_requests.html#get-single-mr

This MR adds the new draft field and deprecates the old field.

PS: It would be great to get a guide on how to properly format this project :)
I'm always struggling to contribute, since the formatting doesn't seem to be idempotent, even with a cargo +nightly fmt.