Add OCM Notification Received Event

[mickenordin]

Summary

OCM is standardizing and expanding the use of notifications and having an event for acting on in apps will be very useful. The immediate use-case will be for an app the implements the proposed federated group sprecification in OCM that relies heavily on notifications.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

I will make a separate pr to nextcloud-documentation when and if this pr is accepted.

AI

I made an AI double check my test case, and made modifications after, but I wrote the code my self.

[AndyScherzinger]

/backport to stable34

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