Some combinations of start_date and frequency/interval values can cause a recurring automation rule to either trigger before its start_date or never trigger. Example repros:
- Configure a recurring automation with hourly recurrence and a start_date several days ahead. What this will do is make the automation start running hourly immediately even though the start_date is several days ahead.
- Configure a recurring automation with a weekly recurrence and a start_date several weeks ahead. This will result in the automation never triggering even after the start_date.
These 2 scenarios share the same cause which is that the automation plugin doesn't use the start_date as the date for the first run and instead uses the frequency/interval values from the current time to calculate the first run date.
This PR fixes this bug by adding an explicit check for start_date and using it as the first run's date if it's ahead of the current time.
Prior to this fix, any change to an automation would reset `pending_automations`, now we only do it if any value related to recurrence (start_date, interval, frequency, execute_at...) has been changed.
It means that any trigger creating `pending_automations` now needs to manage them in the `on_update` callback.
