Fix / implement GH-15287: add a lazy fetch to Pdo\PgSql

Make Pdo\PgSql accept Pdo::setAttribute(PDO::ATTR_PREFETCH, 0) to enter libpq's single row mode.
This avoids storing the whole res

Fix / implement GH-15287: add a lazy fetch to Pdo\PgSql

Make Pdo\PgSql accept Pdo::setAttribute(PDO::ATTR_PREFETCH, 0) to enter libpq's single row mode.
This avoids storing the whole result set in memory before being able to call the first fetch().

close GH-15750

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