Cron / timer triggered components

[radu-matei]

It would be helpful to be able to write components and applications triggered by a cron schedule.

Each component in a "cron" application would be able to define its own schedule.

[radu-matei]

@ecumene has started experimenting with this - ref https://github.com/ecumene/spin/blob/cron/crates/schedule/src/lib.rs#L77

[lann]

Bikeshedding: how about ClockTrigger? Covers both cron-like (specific times of day) and timer-like (intervals, not caring about specific time) cases.

[vdice]

Is this still intended for the upcoming v0.3.0 release? Or should we remove the milestone for now?