new 'json' type, used in Athena Queries what use 'cast (col as json)'

[poolieweb]

When using 'CAST' to a type of 'JSON' in a Athena Query Json type causes an error.

[tejasmanohar]

What Go type is returned for JSON? []byte?

[poolieweb]

Normally []byte , but I struggled with this for a while. Looking at my results and here [https://prestodb.github.io/docs/current/functions/json.html]
When casting to json you get no dictionary keys for the return object, which is kind of expected with row centric SQL, but then this does not allow []byte go unmarshal, unless I'm missing something.
Also I was thinking more in line of keeping to the SQL type primitives then go view point. But this is certainly open discussion, the main point was the err caused by the unmapped json type.

[tejasmanohar]

I like []byte... that's how I've seen jackc/pgx, the postgres driver I use do it, but it would be odd if Go could not unmarshal it. Could you show an example of that?

Also, could you add tests illustrating the expected usage of the JSON type? That's a requirement to merge. Thanks for putting this PR together!