Consider extending cron to push wake ups

Multiple different alerts could be used to wake up a snap.

Examples provided by the Vikings team:
* When something happens on-chain (events log)
* EPNS (Push)
* Server hooks

Currently snaps side-step the issue by entering a polling infinite loop

Start by locating the current cron implementation and the polling infinite loop used by snaps. Review how wake-ups are scheduled today, then determine the scope for event-log, EPNS Push, and server-hook alerts. Done should include an agreed design and implementation path for supporting these wake-up sources.

Written by the indexing model from the issue text.