Pass back the Username directive returned by an AuthenticationProvider
to the calling code, so we may override the user-provided Username
value if the authentication provider passes this back. Useful for
instance in an LDAP scenario where what the user types may not
necessarily be the "username" that is mapped in the system, e.g.
the user providing 'mail' while 'uid' is the "username" value.
Could also then be extensible to other authentication providers
as well, should they wish to do a similar thing.
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