Send device list update EDUs on room join

[kegsay]

Fixes #11374 Tested in matrix-org/complement#706

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[kegsay]

Closing this PR because, for this to be useful, it requires MSC4081 to be included as well. Given we don't have an implementation of MSC4081 then this PR just adds more network traffic without concretely fixing anything.

Why does this PR need MSC4081? It is supposed to guard against the following failure mode:

• Alice joins a room over federation to Bob's server.
• Alice's server goes offline.
• Bob wants to send a message to Alice.

In this scenario, because we do not share device list updates eagerly, Bob's server will not know which devices Alice has, so will need to fetch them. This fetching happens when Bob's client comes online and decides to try to send a message in the room. Here's the problem: to successfully send the message, Bob needs both the device list and one-time keys for each device. But Bob cannot get this because Alice's server is down. If we did MSC4081 we would instead use the fallback key which would've been eagerly sent.