Create dedicated variables for daemon UID, GID

[sed-i]

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Request

It would be handy to have vars such as CRAFT_NON_ROOT_UID and CRAFT_NON_ROOT_GID.

The problem it solves

Currently, the docs state:

Currently, the only supported shared user is daemon (UID/GID 584792). If unset, the user root (UID/GID 0) is selected as the default OCI user.

As a result, we end up with the 584792 literal sprinkled in spread scripts and rockcraft *-override parts.

[OchukoWH]

Hi @sed-i

seems this issue is related to Issue 1291

I would take both up, if still available