Allow hooking to related-pages purge events

The `litespeed_api_purge_post` allows hooking to post purges.

But, dependent on settings, when a post is purged, other pages might be purged as well (front page, blog page, etc.).

There is currently no way to hook to the purge events of these related purges.

Maybe a new hook can be introduced.

Start by tracing the litespeed_api_purge_post hook and the code paths that purge related pages such as the front page or blog page. Determine the intended hook timing and payload for those related purge events; done means consumers can reliably hook each related purge without changing existing post-purge behavior.

Written by the indexing model from the issue text.