Allow verifying encryption keys.

[kevincox]

When end-to-end encryption is enabled there is currently no way for a bridge user to see which encryption key is being used. It would be great if the key fingerprint could be shown to the user so that they can verify if their conversation is secure with the right key on each side. This would be a powerful way to chat over Meta services while verifying that the message contents are end-to-end encrypted by a key controlled by the bridge.