JSON User/Group records: Add properties for UUIDs (#37024)

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.

At this time we are merely adding the properties to the user/group
records. Adding ways to perform lookups by these IDs is left for a
future PR.

See [discussion](https://mastodon.social/@pid_eins/114283987142625086) and
[this comment](https://github.com/systemd/systemd/issues/24032#issuecomment-2745246757).

I'm sure there are wording aspects which could be improved, but I
believe this is a reasonable initial stab at the problem.