MySQL JSON (22174) (#22175)

Pursuant to https://github.com/metabase/metabase/issues/22174.

Make the JSON columns work on MySQL in exactly the same way as they do in Postgres. Bit of a refactor about some bits of Postgres implementation.

Importantly, MariaDB JSON columns work significantly more like SQL Server JSON columns. That is, they aren't JSON columns at all with respect to type, they're text columns - and we don't detect them very well as JSON columns qua JSON columns. So MariaDB is not supported at this time.