Improve consistency in property name for custom reward types

[CaveMobster]

Feature Request

Currently HelixCreateCustomRewardData and HelixCustomReward have an inconsistency. The first has autoFulfill as a property name for should_redemptions_skip_request_queue, and the latter has autoApproved. I think it'd be nice to have them be the same name.

I'd like to update one of them and mark the old name as deprecated if that's okay. I think autoFulfill would semantically be the best name for it, but I'm more than happy to implement whichever is preferred.

[d-fischer]

Fixed this for the next 5.1 prerelease, as deprecations don't usually go into patch versions (I'm trying to be more strict about this)