feat: unified scheduling model

[garethgeorge]

Unify all scheduled operations behind a single proto for defining schedules.

Supported scheduling types are

• time intervals (e.g. the operation runs no more frequently than every x days irrespective of the time of day).
• cron expression e.g. the operation runs as close as possible to the cron specified.

This sets up for adding more scheduled task types e.g. prune on a better defined schedule and also check operations.