Policy server part 1: Actually call the policy server (#18387)

Roughly reviewable commit-by-commit.

This is the first part of adding policy server support to Synapse. Other
parts (unordered), which may or may not be bundled into fewer PRs,
include:

* Implementation of a bulk API
* Supporting a moderation server config (the `fallback_*` options of
https://github.com/element-hq/policyserv_spam_checker )
* Adding an "early event hook" for appservices to receive federation
transactions *before* events are processed formally
* Performance and stability improvements

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
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.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: turt2live <1190097+turt2live@users.noreply.github.com>
Co-authored-by: Devon Hudson <devon.dmytro@gmail.com>

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