Implement EventPayload models for the event types supported by GitHub API

[ryangribble]

GitHub API has currently 37 event payloads that the Activity API can return, and only 8 of these are implemented in Octokit.net!

To support these as first class objects, Octokit needs a response model class added here for each one, and this magic switch statement updated to suit.

We should also add a new unit test and integration test for each payload (although these may fail if they are uncommon events so maybe we can change to unit tests using hardcoded json content from the github api docs).

Related to #1635 (DeleteEvent not supported) and #1645 (proposed workaround to allow unsupported event payloads to be provided as a Json string)

List of current event types

• CommitCommentEvent
• CreateEvent
• DeleteEvent
• DeploymentEvent
• DeploymentStatusEvent
• DownloadEvent
• FollowEvent
• ForkEvent
• ForkApplyEvent
• GistEvent
• GollumEvent
• InstallationEvent
• InstallationRepositoriesEvent
• IssueCommentEvent
• IssuesEvent
• LabelEvent
• MarketplacePurchaseEvent
• MemberEvent
• MembershipEvent
• MilestoneEvent
• OrganizationEvent
• OrgBlockEvent
• PageBuildEvent
• ProjectCardEvent
• ProjectColumnEvent
• ProjectEvent
• PublicEvent
• PullRequestEvent
• PullRequestReviewEvent
• PullRequestReviewCommentEvent
• PushEvent
• ReleaseEvent
• RepositoryEvent
• StatusEvent
• TeamEvent
• TeamAddEvent
• WatchEvent

[Hollywood]

I can hop on this one. @ryangribble

[ryangribble]

Just noting that StatusEvent will be implemented in #1732 once merged

[github-actions]

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