Move legacy custom search framework to a core extension

I think we have a medium term goal of disentangling the custom search framework out of core search code - this would allow changes like https://github.com/civicrm/civicrm-core/pull/19143 without adding new custom search hackery to the core search classes. There has been a bit of chicken and egg - ie it didn't make sense to separate out the inappropriately shared classes (like the search controller) without extensionising & vice versa.

However, doing this split turns out to be fairly easy in all places except for GroupContactCache where I think we would need a new hook to separate out