abstract 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 HashSaltStyle
user - the user to get the salt for. This paramter is needed, if the HashSaltStyle#COLUMN is declared to be used
Return
null in case of HashSaltStyle#NO_SALT the salt of the user or a defined external salt