Fix #233 Add limit pushdown to attached queries

[noahisaksen]

Implements limit pushdown #233

Gets applied on all postgres attached queries, if preferred can be applied only when an option like experimental pushdown is set, however in this implementation triggers the same as the mysql_scanner limit pushdown implementation.

This change greatly improves performance for queries that include a LIMIT.

What it does:

select bar from attached_pg.foo limit 1;

Previously

COPY (SELECT "bar" FROM "attached_pg"."foo" ) TO STDOUT (FORMAT "binary");

Now

COPY (SELECT "bar" FROM "attached_pg"."foo" LIMIT 1) TO STDOUT (FORMAT "binary");

---

I am not sure how a specific test for this would look like as I am not sure there is a way to verify generated SQL. A generic limit test was added.

---

Works for LIMIT and OFFSET. If an ORDER_BY is present it will not be added.