Fix Socrata previews returning null

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):

```sql
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:

```sql
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
```

splitgraph/hooks/data_source/fdw.py

@@ -235,9 +235,10 @@ def _preview_table(self, schema: str, table: str, limit: int = 10) -> List[Dict[
         result_json = cast(
             List[Dict[str, Any]],
             self.engine.run_sql(
-                SQL("SELECT row_to_json(t.*) FROM {}.{} t LIMIT %s").format(
-                    Identifier(schema), Identifier(table)
-                ),
+                SQL(
+                    "WITH p AS MATERIALIZED(SELECT * FROM {}.{} LIMIT %s) "
+                    "SELECT row_to_json(p.*) FROM p"
+                ).format(Identifier(schema), Identifier(table)),
                 (limit,),
                 return_shape=ResultShape.MANY_ONE,
             ),

test/splitgraph/ingestion/test_socrata.py

@@ -423,3 +423,7 @@ def test_socrata_data_source_introspection_smoke(local_engine_empty):
     schema, params = result["some_table"]
     assert len(schema) > 1
     assert params == {"socrata_id": "8wbx-tsch"}
+
+    preview_result = data_source.preview(tables=result)
+    assert len(preview_result["some_table"]) == 10
+    assert preview_result["some_table"][0][":id"] is not None