open fun getSalt(user: User): String
Retrieve the salt. The source of the salt can be the external salt or the propriate column of the given user, depending on the defined io.vertx.reactivex.ext.auth.mongo.HashSaltStyle
user - the user to get the salt for. This paramter is needed, if the is declared to be used
Return
null in case of the salt of the user or a defined external salt