fix: add better explain for merge into

[JackTan25]

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Summary

Summary about this PR

mysql> explain MERGE INTO salaries USING (SELECT * FROM employees) as employees ON salaries.employee_id = 
employees.employee_id WHEN MATCHED AND employees.department = 'HR' THEN UPDATE SET salaries.salary = 
salaries.salary + 1000.00 WHEN MATCHED THEN UPDATE SET salaries.salary = salaries.salary + 500.00 WHEN NOT 
MATCHED THEN INSERT (employee_id, salary) VALUES (employees.employee_id, 55000.00);
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| explain                                                                                                                                                                     |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| MergeInto:                                                                                                                                                                  |
| target_table: default.default.salaries                                                                                                                                      |
| ├── matched update: [condition: eq(employees.department (#2), 'HR'),update set salary = plus(salaries.salary (#4), 1000.00)]                                                |
| ├── matched update: [condition: None,update set salary = plus(salaries.salary (#4), 500.00)]                                                                                |
| ├── unmatched insert: [condition: None,insert into (employee_id,salary) values(CAST(employees.employee_id (#0) AS Int32 NULL),CAST(55000.00 AS Decimal(10, 2) NULL))]       |
| └── HashJoin: RIGHT OUTER                                                                                                                                                   |
|     ├── equi conditions: [eq(salaries.employee_id (#3), employees.employee_id (#0))]                                                                                        |
|     ├── non-equi conditions: []                                                                                                                                             |
|     ├── LogicalGet                                                                                                                                                          |
|     │   ├── table: default.default.salaries                                                                                                                                 |
|     │   ├── filters: []                                                                                                                                                     |
|     │   ├── order by: []                                                                                                                                                    |
|     │   └── limit: NONE                                                                                                                                                     |
|     └── EvalScalar                                                                                                                                                          |
|         ├── scalars: [employees.employee_id (#0), employees.employee_name (#1), employees.department (#2)]                                                                  |
|         └── LogicalGet                                                                                                                                                      |
|             ├── table: default.default.employees                                                                                                                            |
|             ├── filters: []                                                                                                                                                 |
|             ├── order by: []                                                                                                                                                |
|             └── limit: NONE                                                                                                                                                 |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
20 rows in set (0.11 sec)
Read 0 rows, 0.00 B in 0.005 sec., 0 rows/sec., 0.00 B/sec.

• Closes #issue
  Explain Replace, Merge into etc return empty info #13999

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test

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This change is 

[xudong963]

You can add a explain merge into test

[leiysky]

It's better to move the test to this folder, which is for the explain tests running in standalone mode.