third_party/heimdal_build: Add SFU source file

third_party/heimdal_build: Add SFU source file

This is an adaptation to Heimdal:

commit 0287558838de79313e38026d2f0905ffc987d0b8
Author: Luke Howard <lukeh@padl.com>
Date:   Fri Dec 24 13:49:55 2021 +1100

    kdc: move Services for User implementation out of krb5tgs.c

    Move the Services for User (SFU/S4U) implementation -- protocol transition and
    constrained delegation -- into its own compilation unit, with an interface that
    only takes an astgs_request_t, so it can be easily factored out into a plugin
    module in the future.

    This refactoring is also careful to update all client names in the request
    structure after the SFU/S4U validation has successfully completed.

NOTE: THIS COMMIT WON'T COMPILE/WORK ON ITS OWN!

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14995

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