Effects: set user id, group id of in-container process

[roberth]

Some software - notably nix - may behave differently when it's uid 0.

Changing the "virtual" uid seems possible with OCI Container process.user.uid = <json int> (but docs are sparse?)
Get the number from derivation magic attribute __hci_effect_uid or something. (Don't think it had to prefix anything yet)
Same for gid. Maybe other such options as well?
Add __hci_effect_minAgentVersion?

Also / when done

• handle the version check on the backend scheduler to solve the forward/backward compat problem instantly
• create friendly aliases for these parameters in hercules-ci-effects