Add duckdb.log_pg_subplans GUC setting

[Y--]

When enabled, dump the query and plan executed by PG during DuckDB execution.

Example:

y=# SET duckdb.log_pg_subplans = true;
SET
y=# SET duckdb.force_execution = true;
SET
y=# select count(*) from f;
INFO:  Query:
SELECT COUNT(*) FROM public.f
INFO:  Plan:
Aggregate  (cost=1693.00..1693.01 rows=1 width=8)
  ->  Seq Scan on f  (cost=0.00..1443.00 rows=100000 width=0)

 count
--------
 100000
(1 row)