Add search parameters to EXPLAIN (SETTINGS) - #697

EXPLAIN (SETTINGS) SELECT * FROM items ORDER BY embedding <-> '[3,1,2]' LIMIT 5;
                                       QUERY PLAN                                        
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 Limit  (cost=8.02..12.04 rows=2 width=48)
   ->  Index Scan using items_embedding_idx on items  (cost=8.02..12.04 rows=2 width=48)
         Order By: (embedding <-> '[3,1,2]'::vector)
 Settings: hnsw.iterative_search = 'relaxed_order'