Fix Socrata previews returning null

[mildbyte]

It looks like row_to_json(t.*) makes Multicorn think we're requesting no
columns from the FDW (this could be related to our Multicorn changes, but it
doesn't happen with the LQFDW):

explain SELECT row_to_json(p.*) FROM sg_tmp_67cecbc993a0738c2f815e80169dafd3.some_table p LIMIT 10

Limit  (cost=20.00..20.02 rows=10 width=32)
  ->  Foreign Scan on some_table p  (cost=20.00..237.38 rows=94954 width=32)
        Multicorn: Socrata query to data.cityofnewyork.us
        Multicorn: Socrata dataset ID: 8wbx-tsch
        Multicorn: Query:
        Multicorn: Columns:
        Multicorn: Order: :id

This query returns JSON objects full of NULLs.

Rewrite the preview query to first get the first 10 rows as a materialized CTE,
then use row_to_json on the result, which seems to fix the issue:

explain SELECT t.* FROM sg_tmp_67cecbc993a0738c2f815e80169dafd3.some_table t LIMIT 10

Limit  (cost=20.00..33620.00 rows=10 width=3360)
  ->  Foreign Scan on some_table t  (cost=20.00..319045440.00 rows=94954 width=3360)
        Multicorn: Socrata query to data.cityofnewyork.us
        Multicorn: Socrata dataset ID: 8wbx-tsch
        Multicorn: Query:
        Multicorn: Columns: `last_date_updated`,`license_type`,`last_time_updated`,`vehicle_license_number`,`wheelchair_accessible`,`vehicle_year`,`permit_license_number`,`order_date`,`certification_date`,`base_address`,`base_name`,`reason`,`hack_up_date`,`expiration_date`,`vehicle_vin_number`,`base_number`,`base_telephone_number`,`dmv_license_plate_number`,`website`,`active`,`base_type`,`veh`,`name`
        Multicorn: Order: :id

explain with p as materialized
    (SELECT * FROM sg_tmp_67cecbc993a0738c2f815e80169dafd3.some_table LIMIT 10)
select row_to_json(p.*) from p

CTE Scan on p  (cost=33620.00..33620.22 rows=10 width=32)
  CTE p
    ->  Limit  (cost=20.00..33620.00 rows=10 width=3360)
          ->  Foreign Scan on some_table  (cost=20.00..319045440.00 rows=94954 width=3360)
                Multicorn: Socrata query to data.cityofnewyork.us
                Multicorn: Socrata dataset ID: 8wbx-tsch
                Multicorn: Query:
                Multicorn: Columns: `last_date_updated`,`license_type`,`last_time_updated`,`vehicle_license_number`,`wheelchair_accessible`,`vehicle_year`,`permit_license_number`,`order_date`,`certification_date`,`base_address`,`base_name`,`reason`,`hack_up_date`,`expiration_date`,`vehicle_vin_number`,`base_number`,`base_telephone_number`,`dmv_license_plate_number`,`website`,`active`,`base_type`,`veh`,`name`
                Multicorn: Order: :id