Implement MSC3051 to support multiple relations per event

[matrixbot]

This issue has been migrated from #16111.

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It seems there's considerable interest in MSC3051 - scalable relations (multiple relations per event in an array) as the limits of the current model become more and more apparent. This is a WIP attempt to implement it (there's no tests yet).

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[laddhoffman]

@deepbluev7 Do you know the latest branch for multi relations? https://github.com/chayleaf/synapse/tree/msc3051-rebase is referenced in matrix-org/synapse#16111