Add timestamp massaging to `/createRoom`

[matrixbot]

This issue has been migrated from #15346.

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As propsed by MSC3997, following in the footsteps of MSC3316's timestamp massaging query parameter for /send?ts=123, add the same thing to the /createRoom endpoint

POST /_matrix/client/v3/createRoom?ts=123

Why

When writing end-to-end tests, it's useful to have your room appear as though it was created back in time before your messages were sent and to have stable/consistent timestamps. If you start using the timestamp massaging with messages, it can appear as though those events occurred before the m.room.create and other primordial events.

So this issue, proposes being able to modify the origin_server_ts of all of the primordial room creation events.

This could also apply to other endpoints (/join, /ban) but I'm mostly concerned with the /createRoom endpoint because that's the hairy area in my Matrix Public Archive tests at the moment. And /ban, etc can be emulated with the normal /send endpoint and sending the appropriate events.

/join can only be emulated for local joins as joins against rooms the server doesn't know about doesn't work since there is no way to specify via servers`. Created MSC3998

Todo

• Create MSC -> MSC3997: Add timestamp massaging to /createRoom matrix-org/matrix-spec-proposals#3997
  • Also see MSC3998