Allow 3rd party rules check_event_allowed callback to function for room version "12" creation events

[jason-famedly]

The change to room version "12" removes that a room ID is included in the room creation event, and is instead generated based on the event hash of said event. This seems to break the check_event_allowed() callback for this room version specifically for creation events due to an assert in the function that expects a room ID to be present.

Specifically ignore that assertion for creation events on rooms that support MSC4291(Room ID's as hashes) to fix this. This also needs to be checked for the correct assumptions about MSC4242 taken in it's second half(that creation events do not contain prev_state_events in the same way that they do not include prev_events)

Note this contains a commit that should be reverted before merging, that overrides the test so room version "12" is used

💡 This is the command line to run the specific test:
poetry run trial tests.rest.client.test_third_party_rules.ThirdPartyRulesTestCase.test_legacy_check_event_allowed

Failure looked like this before:

[ERROR]
Traceback (most recent call last):
  File "/home/jason/dev/famedly/synapse-upstreaming/tests/rest/client/test_third_party_rules.py", line 394, in test_legacy_check_event_allowed
    "/_matrix/client/r0/rooms/%s/send/m.room.message/1" % self.room_id,
builtins.AttributeError: 'ThirdPartyRulesTestCase' object has no attribute 'room_id'

tests.rest.client.test_third_party_rules.ThirdPartyRulesTestCase.test_legacy_check_event_allowed
===============================================================================

(There is another error in this TestCase when using room version "12", but it is unrelated to this and will be handled separately when fixing the tests themselves)

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.
• Code style is correct (run the linters)