[META] Add support for JSON field queries in entity queries

I'm all with catch (#5) in that this needs workable steps. Started to code a json_map_field (will share soon, collaborators welcome), but realized that this is currently impossible, as serialization of the builtin map field relies on a megadeth kittenkiller hack.

Started some work in the related (9y old!) issue.

The title of this issue says "entity queries", but to support this in entity queries we must first support this in Drupal's database API itself. I opened ✨ Add support for JSON field queries in database API Active as a child issue of this meta for discussing and implementing that step.

I'm going to close this as a duplicate since due to all the tight coupling with the database API changes over in ✨ Add "json" as core data type to Schema and Database API Needs work .