State event to describe bridge constraints for messages

[Half-Shot]

We currently have no way to indicate to a user how "well" their message will bridge through to a protocol. Factors like support for replies, images, or even markdown might not exist on a bridge and we need a way to tell the user that their message will be sent in a lesser form.

This also ties into the complaints that the IRC bridge does nothing to limit the size of messages which tends to piss off some channel ops, understandably.

I'd propose a m.bridge.constraints state event keyed off the bridge's userid. The intention would be to hint to the client that their messages may be sent incomplete in some way. However, this would not stop the user from sending messages and bridges should still be prepared to handle them.