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!