Visual hash of this server

Returns a PNG of the visual hash corresponding to this server. This visual hash is used during the registration process (optional), for the user to verify that (s)he is registering with the right server in the nextAuth app. For single-server nextAuth-enabled apps (white label or mobile SDK), the public key of the server is typically pinned within the app and hence this visual hash is not displayed to the user. Required permission: 'sessions' or 'servers'.