JSON User/Group records: Add properties for UUIDs

JSON User/Group records: Add properties for UUIDs

It is useful to have stable and unique identifiers for a security principal.
The majority of identitiy management systems in use with Unix systems today
(e.g. Active Directory objectGUID, FreeIPA ipaUniqueID, Kanidm UUIDs) assign
each account and group a unique UUID and exposing that to applications allows
them to refer to accounts in a stable manner.

This change does not implement user or group lookup by UUID; that is left for
a later PR.