This specification defines the informational
capability. If present, it indicates that the connection to the server cannot
be completed unless the clients authenticates, typically via SASL. Note, the
absence of this capability does not indicate that connection registration can
be completed without authentication; it may be disallowed due to specific
properties of the connection (e.g. an untrustworthy IP address), which will be
indicated instead by
FAIL * ACCOUNT_REQUIRED.
Clients MUST NOT request
soju.im/account-required; servers MUST reject any
CAP REQ command including this capability.