JSON_TABLE: extract document fragment into JSON column

Currently, JSON_TABLE doesn't allow one to extract a JSON "subdocument" into a JSON column.

Both the SQL Standard and MySQL-8 provide this capability. This task is about implementing it.

I was sent a patch with MySQL-8 syntax support, so I assume we're going to implement MySQL-8 compatible syntax, which is just to allow specifying column type as JSON. For example:

select * from

  json_table('{"foo": [1,2,3,4]}',

             '$' columns( jscol json path '$.foo')

             ) as T;

should emit [1,2,3,4].