Use varchar for MySQL/PostgreSQL native JSON fields.

[stewartpark]

Fixes #5649
Also mentioned in #5989 #6614

We noticed that native JSON fields are not supported for our MySQL/PostgreSQL databases.

The changes introduce a new behavior where the field types are not in the cases, it just assumes the type as varchar, which is pretty reasonable, giving back users the chance to parse/deal with the data easily in case of missing native support. But open to suggestions!

[electrum]

For JSON, we'd rather map these to Presto's JSON type. However, I started working on that a while back and it turned out to be rather complex, so this is a reasonable first step.

What happens with inserts into an existing table with JSON or other types?

[xborderx]

Any update on this issue?
Thanks!

additionally, my use case is simply importing data from psql to Presto and I don't worry about exporting from Presto to psql. So I really just a readonly support for jsonb.

[gseva]

We're interested in this also. If we could access json/jsonb columns as text, it would be already very useful.

[dowjones226]

yeah, any updates on this?

[stewartpark]

Since it's been getting some interest, I feel obligated to update you guys here. This was buried with some other big task I had at the time and I failed to update the PR. So the last time I worked on this, I had a problem inserting like @electrum suggested. Didn't explore options to solve this, so I'll close this PR and repoke(or let others poke) at this when I have some allocated time for this.