MSC4242: State DAGs (inbound joins) (#20194)

Process inbound requests to join a state DAG room.

Split out from https://github.com/element-hq/synapse/pull/19425

Part of a series of 5x PRs to land the federation part of MSC4242
(https://github.com/element-hq/synapse/pull/19718,
https://github.com/element-hq/synapse/pull/20127,
https://github.com/element-hq/synapse/pull/20133, inbound-joins (this
PR), inbound-pulls).

Has some Complement tests:
```
PASS TestMSC4242JoinLeaveRejoinPublicRoom 9.19s
PASS TestMSC4242JoinPublicRoomWithRejectedStateDAGEvents 0.88s
```

### Pull Request Checklist

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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`.".
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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: Eric Eastwood <madlittlemods@gmail.com>
Co-authored-by: Eric Eastwood <erice@element.io>

scripts-dev/complement.sh

@@ -287,6 +287,7 @@ main() {
     ./tests/msc4306
     ./tests/msc4222
     ./tests/msc4429
+    ./tests/msc4242
   )
 
   # Export the list of test packages as a space-separated environment variable, so other