[WP Super Cache] Add specific time scheduling to Preload

[zeus2611]

Fixes #46754

Proposed changes:

• Feature: Added a "Scheduler" option to the Preload settings, allowing users to choose between "Timer" (Interval) and "Clock" (Specific Time).
• UI: Ported the "Clock" UI selector from the Garbage Collection settings to the Preload tab for consistency.
• Logic: Updated wp-cache-preload.php to correctly schedule the wp_cache_preload_hook event at a specific daily time when "Clock" mode is selected.
• Goal: This allows server administrators to schedule resource-intensive preloading during off-peak hours (e.g., 03:00 AM) to minimize impact on site performance.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

N/A (Community contribution fixing a GitHub issue).

Does this pull request change what data or activity we track or use?

No. This change only affects the internal scheduling of cache preloading.

Testing instructions:

1. Setup: Install and activate WP Super Cache (or use the local dev environment).
2. Navigate: Go to Settings > WP Super Cache > Preload tab.
3. Verify UI: Confirm there is now a "Scheduler" section with "Timer" and "Clock" radio buttons.
4. Test Clock Mode:
   • Select "Clock".
   • Enter a time shortly in the future (e.g., if it is 14:00, enter 14:05).
   • Click Save Settings.
   • Verification: Use a plugin like WP Crontrol to verify that wp_cache_preload_hook is scheduled for 14:05 today (or tomorrow if the time passed).
5. Test Timer Mode:
   • Select "Timer".
   • Enter an interval (e.g., 600 minutes).
   • Click Save Settings.
   • Verification: Confirm the hook is scheduled to run based on the interval.

[github-actions]

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