feat: Support merge_group events

[felipefrancisco]

Description

Important

DISCLAIMER: I'm not a Java developer - This PR is based on the changes implemented in #1316, which seemed quite simple to do, so I'm proposing these changes for review.

---

This PR adds support for merge_group events fired by merge queues, please see the official documentation for details:

• Github Blog post
• merge_group event definition in workflow docs

I'm submitting this PR since I'm using jenkinsci/github-plugin, which is failing to accept these valid events fired by Github. It does an event-check here based on org.kohsuke.github.GHEvent (defined by this project) and, since merge_group is not a supported event, it fails with "Hook should contain event type".

Here's an example of what I see on Github Settings > Webhook on my end when Jenkins receives that payload:

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[felipefrancisco]

Hey @bitwiseman, apologies for the direct ping - I noticed that you're an active contributor to this project, so I was hoping you could kindly have a look through this very short PR in case you get a chance.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
src/main/java/org/kohsuke/github/GHEvent.java	100.00% <100.00%> (ø)

📢 Thoughts on this report? Let us know!.

[felipefrancisco]

Thank you for the speedy feedback @bitwiseman 🙏🏽