[FEATURE] defpoll support for polling at a cron-like instants rather than a specific interval

[nagisa]

Description of the requested feature

It would be neat if defpoll supported in addition to the usual interval period a cron-like ability to specify polling at specific instants.

Right now doing something like this requires writing a script and using deflisten which is of course super-general, but ain't nobody is doing that for a clock-like thing (bash doesn't exactly have a sleep_until alike.)

Proposed configuration syntax

;; calls date only when the clock hits 0th second.
(defpoll variable :at "0 * * * * *" "date '+%H:%M'")

Additional context

Implementation wise sleep_until-like function along with the cron crate could be used.