pylibs: add string_is_guid() helper.

pylibs: add string_is_guid() helper.

In various places we use regular expressions to check for GUID-ness,
though typically we don't match GUIDs with uppercase hex digits when
we really should.

If we centralise the check, we have more chance of getting it right.

Pair-programmed-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Thu Feb 29 02:38:07 UTC 2024 on atb-devel-224