Add Sliding Sync `/sync` endpoint

[MadLittleMods]

Add Sliding Sync /sync endpoint.

Based on MSC3575: Sliding Sync

Dev notes

• Pydantic validation in Synapse: https://github.com/matrix-org/synapse/milestone/9
• https://docs.pydantic.dev/latest/api/types/
• https://docs.pydantic.dev/2.0/usage/types/list_types/

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Avoid shadowing arguments - disallow reassignment - masking existing variables:

• https://github.com/matrix-org/synapse/pull/10439#discussion_r673819501
• https://github.com/wemake-services/wemake-python-styleguide/issues/2128
• https://github.com/python/mypy/issues/11076

Pull Request Checklist

• [ ] Pull request is based on the develop branch
• [x] 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.
• [ ] Code style is correct (run the linters)