Add support for MERGE INTO/ON CONFLICT to Postgres

[Mytherin]

Implements #189
Implements #278
Implements #309

This PR adds support for the MERGE INTO / ON CONFLICT clauses to the Postgres scanner, e.g. this now works:

D ATTACH 'postgres:dbname=postgresscanner' AS pgdb;
D CREATE OR REPLACE TABLE pgdb.my_tbl(id INTEGER PRIMARY KEY, val VARCHAR);
D INSERT INTO pgdb.my_tbl VALUES (42, 'hello');
D INSERT OR REPLACE INTO pgdb.my_tbl VALUES (42, 'world');
D FROM pgdb.my_tbl;
┌───────┬─────────┐
│  id   │   val   │
│ int32 │ varchar │
├───────┼─────────┤
│  42   │ world   │
└───────┴─────────┘
D INSERT INTO pgdb.my_tbl VALUES (42, 'hello again') ON CONFLICT DO UPDATE SET val=excluded.val;
D FROM pgdb.my_tbl;
┌───────┬─────────────┐
│  id   │     val     │
│ int32 │   varchar   │
├───────┼─────────────┤
│  42   │ hello again │
└───────┴─────────────┘
MERGE INTO pgdb.my_tbl USING (VALUES (42, 'world again'), (43, 'new world')) t(id, val) USING (id)
  WHEN MATCHED THEN UPDATE
  WHEN NOT MATCHED THEN INSERT;
D FROM pgdb.my_tbl;
┌───────┬─────────────┐
│  id   │     val     │
│ int32 │   varchar   │
├───────┼─────────────┤
│    43 │ new world   │
│    42 │ world again │
└───────┴─────────────┘

[karthik-isw]

Thanks a lot.

[dineshmarimu2]

@Mytherin Which version of duckdb has this upsert feature postgres server implemented? I don't see the milestone in the issue populated. Nor is there any documentation in the postgres extension that explicitly state that this upsert is possible with postgres.