Missing `SponsorshipEvent` in events types

[jpmcb]

There appears to be a new event type, the SponsorshipEvent as defined by the GitHub docs:

https://docs.github.com/en/rest/overview/github-event-types?apiVersion=2022-11-28#sponsorshipevent

This doesn't seem to be implemented:

undefined: github.SponsorshipEvent

and I don't see it in event_types.go: https://github.com/google/go-github/blob/master/github/event_types.go

I'd be happy to pick this up 👍🏼