CVE-2022-32743 s4/dsdb/util: Add dsdb_msg_get_single_value()

CVE-2022-32743 s4/dsdb/util: Add dsdb_msg_get_single_value()

This function simulates an add or modify operation for an ldb message to
determine the final value of a particular single-valued attribute. This
is useful when validating attributes that should stay in sync with other
attributes, such as servicePrincipalName and dNSHostName.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14833

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>