Implement EDUs to AppServices for MSC2409

[tcpipuk]

MSC2409 specifies a way for homeservers to transmit EDUs to AppServices so that bridges can be aware of read receipts, typing, presence, etc.

Currently bridges are able to send them to us, but we're not able to send the same in return, so bridged chats remain unread on the bridged service, for example.

I know the current AppService implementation is a challenge so would probably require a fair bit of work, but this feature currently works really well in Synapse, so I wanted to get an issue logged for the future so users with bridges can have an even better experience in Conduwuit!

[girlbossceo]

#642 implemented read receipt and typing indicator sending EDUs which are the two primary EDUs people want in bridges.

Presence might be a bit more involved, but in theory should be easier because presence is not scoped to rooms or anything, so there shouldn't need to be any kind of check for things like if user is related to an appservice or shares a room with an appservice. Presence is just whether a server shares a room with another server. Also presence doesn't really seem that useful to appservices, local presence might be more beneficial to them anyways.

To-device events still needs to be implemented so E2EE bridging works more reliably/properly.