Proposal - new core extension for scheduled reminders using criteria from search-kit

This replaces #2267 (closed) as a way to address the same issue - ie our search criteria for scheduled reminders is way too brittle due to the inflexible schema. For example it is not possible to use contributions > x amount or contributions from campaign y.

Given that we Apiv4 / search kit is the way we are going with search criteria it makes sense to use these as criteria for the reminder search too to replace (in the distant future) the existing fields.

#2267 (closed) proposed adding api params & api entity to the scheduled reminder table but I now think it makes more sense as an extension - which I have been experimenting with.

This extension does not have to go into core - it could be something I just maintain to the degree I see fit.

However, the main reason we would consider it as a core extension is that it could eventually replace the existing scheduled reminder form (which would fit our LEXIM plan to get off quick forms and onto everything being apiv4 based). The hope is not to replace the additional functionality in CiviRules for people who want that but to reduce pain points in the scheduled reminder code that is already in core and as a path away from maintaining the relevant quickforms.

Note that if we proceed with this as a core extension I will PR it in once it hits 'alpha' but we would not enable by default and we can have a discussion at that point as to whether it should be hidden or not depending on how mature it is.

I note @totten's response to the question as to whether it should go into core rather seemed to hinge on whether the UI is any good - and since it's not yet written we can't address that at this stage.