Serdes v2: Refactor entire serdes flow to use real Toucan, not low level (#25981)

Previously, some automatic behavior was causing problems in serdes.
(The worst example is generating an `entity_id` on insert while
deserializing an entity we don't own - if deserialized again it would be
duplicated.)

A whole cascade of design choices fell out of this problem:
- `insert!` would generate `entity_id`s, so use `simple-insert!`
- But `simple-insert!` doesn't convert eg. MBQL `:definition` maps back
  into JSON strings
- We got the maps as Clojure data and not JSON strings because `select`
  and `simple-select` both run `post-select` and parse the JSON.
- So we ended up with a raw query on the select side and `simple-insert!`
  on the storage side.

This change unwinds that whole stack, and instead uses a dynamic
var to suppress the few pieces of `pre-insert` and `pre-update` logic
that causes problems.

The end result is much cleaner, and much more consistent with the rest
of Metabase's backend logic.