Add passthrough authentication support for Xpand LDAP

Xpand is adding authentication to LDAP directly within Xpand. The users are only stored in LDAP. Xpand requires the client cleartext password to authenticate against LDAP.

Add support for MaxScale to handle unknown user by connecting immediately through Xpand, without assuming the authentication will succeed. (Currently, MaxScale can support unknown user, but it assumes the connection will be successful and returns success to the client immediately. Connection then get deferred to when first SQL is executed).

This is needed for support for LDAP in Xpand which is slotted for Transylvania release, end of 2Q2023.