Add GitHubEvent marker interface

[hashtagchris]

Closes https://github.com/google/go-github/issues/2462

Remaining work:

• [ ] Add comments
• [ ] Fix test generation (destination: github-marker-methods_test.go)

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[gmlewis]

Will close this PR as abandoned in a week or so if no further activity.

[gmlewis]

Closing PR as abandoned.