opt: Add execbuilder support for Delete operator

Build the deleteNode for the CBO Delete operator.

Also, fix bug in processSourceRow, which was incorrectly returning mutation
columns. Only return the subset of sourceVals that were requested.

Release note: None

pkg/sql/logictest/testdata/logic_test/delete

@@ -153,13 +153,13 @@ statement ok
 INSERT INTO unindexed VALUES (1, 9), (8, 2), (3, 7), (6, 4)
 
 query II
-DELETE FROM unindexed WHERE k > 1 AND v < 7 ORDER BY v DESC RETURNING v,k
+DELETE FROM unindexed WHERE k > 1 AND v < 7 ORDER BY v DESC LIMIT 2 RETURNING v,k
 ----
 4  6
 2  8
 
 query II
-DELETE FROM unindexed ORDER BY v RETURNING k,v
+DELETE FROM unindexed ORDER BY v LIMIT 2 RETURNING k,v
 ----
 3  7
 1  9