Add support for relay user distinguishers (⭕️⬛️)

Telegram bridge has this neat feature that adds unique emojis to usernames of users that are bridged via relay:

https://github.com/mautrix/telegram/blob/master/mautrix_telegram/example-config.yaml#L462C32-L462C80

It makes the messages look distinct when there's a lot of messages that all come from the relay bot user.

It would be neat to have this in the Whatsapp bridge, as well!