Bug: COPY FORMAT JSON includes generated columns unlike text/CSV

Hi hackers,

COPY TO with FORMAT json includes generated columns in the output,
while text and CSV formats correctly exclude them.  Virtual generated
columns appear as null (since they aren't materialized), and stored
generated columns appear with their values  Attached a patch to address
this.

Repro:

    CREATE TABLE t (id int, a int,
        s int GENERATED ALWAYS AS (a * 10) STORED,
        v int GENERATED ALWAYS AS (a * 100) VIRTUAL);
    INSERT INTO t (id, a) VALUES (1, 5);

    COPY t TO STDOUT;
    COPY t TO STDOUT WITH (FORMAT csv, HEADER);
    COPY t TO STDOUT WITH (FORMAT json);
CREATE TABLE
INSERT 0 1
1       5
id,a
1,5
{"id":1,"a":5,"s":50,"v":null}

After the fix:
COPY t TO STDOUT WITH (FORMAT json);
{"id":1,"a":5}

Thanks,
Satya