Add no-op handler for GitHub Marketplace events

[eleftherias]

Summary

In order to list our GitHub App on the GitHub Marketplace, we need a webhook handler for Marketplace events.
This is a no-op handler because we don't need to react to marketplace events (e.g. purchases).

Fixes #3063

Change Type

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• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

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Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.