s4:kdc: Add utility functions for AD claims

get_claims_for_principal() is a new function that creates a claims blob
for a principal based on attributes in the database.

It's not hooked into the KDC yet, so this entails no change in
behaviour.

Constructed claims and certificate claims are not supported yet.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
