Enhancement:Time specific preload in wp-super cache

### Impacted plugin

Super Cache

### What

Chose a specific time to preload the cache instead of time intervals

### How

Hi, is there a way to set the preload option to a specific time as there is on the garbage collection instead of just time intervals? I think it would be a good option to schedule the preload with other automated tasks on the server to be in the most optimal order and with the least traffic on the site.

Start by locating the preload option and the existing garbage-collection scheduling path in Super Cache. Determine how a specific clock time could coexist with the current interval setting and server automation; done means users can configure a preload time and the cache refresh runs at that scheduled time without disrupting existing behavior.

Written by the indexing model from the issue text.