chore: use webhook event fixtures and types provided by octokit

[remmycat]

Changes:

• Use event types by octokit, remove the existing github types.
• Add more exhaustive typescript checks to the metrics (e.g. the metricsCondition checks that the event we got satisfies the exact type we assume to receive in the metric handler)
• Use actual event mocks for the metricsCondition tests
• Add snapshot tests for all metrics based on the sample events by octokit (sorry for the 50000 added lines 😅 Theoretically we could skip the snapshot of the input for every run and somehow test that separately, but I think when debugging a failed snapshot it's very valuable to have the exact input it used above, and to get per-test feedback if we update the fixture package)
• Make sure the code review involvement metric explicitly reject unmerged PRs (I'm not sure if this is exactly what we want long-term but it is at least the current behaviour, since an unmerged PR crashed the metric handler).