on_time trigger with template at Time component

**Describe the problem you have/What new integration you would like**

Many automations want timed events. The time can be set in the Home Assistant dashboard and the set time would be stored in the ESP flash (EEprom).
**Please describe your use case for this integration and alternatives you've tried:**

Now the on_time trigger can only get hard-coded time (hours, minutes, seconds, months, cron, etc...), meaning that the time values are fixed in ESP, not changeable by a variable, thus from the Home Assistant Dashboard interface.
**Additional context**

The best solution would be if the on_time trigger could be a template for all configuration variables.
So, for example, in the case of the irrigation controller I'm developing now, watering would start at the time and on the days set in Home Assistant, even if Home Assistant is not available, and would remain at the set value even in the event of a power failure.

Start by reviewing the ESPHome on_time trigger and how its configured time values are handled when the device is offline. Then examine the existing Home Assistant dashboard configuration flow and ESP flash/EEPROM persistence mentioned in the request; done means dashboard-set times and days remain usable after Home Assistant unavailability and power loss.

Written by the indexing model from the issue text.