Adds `eval_offset` attribute for Groups.
If specified, Group will be evaluated at the exact time offset on the range of [0...evaluationInterval].
The setting might be useful for cron-like rules which must be evaluated at specific moments of time.
https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3409
Signed-off-by: Haley Wang <pipilong.25@gmail.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>
