Display reorg type in admin show ddl jobs for multi-schema change

[tangenta]

Enhancement

#36168 supports displaying the sub-job in ADMIN SHOW DDL JOBS, and #38733 supports displaying the reorg type for adding index job. However, when we use multi-schema change to add index, ADMIN SHOW DDL JOBS only shows it is "sub-job", but no reorg type:

mysql> alter table t add index idx(a), add column b int;
Query OK, 0 rows affected (0.52 sec)

mysql> admin show ddl jobs 1;
+--------+---------+------------+---------------------------------+--------------+-----------+----------+-----------+---------------------+---------------------+---------------------+--------+
| JOB_ID | DB_NAME | TABLE_NAME | JOB_TYPE                        | SCHEMA_STATE | SCHEMA_ID | TABLE_ID | ROW_COUNT | CREATE_TIME         | START_TIME          | END_TIME            | STATE  |
+--------+---------+------------+---------------------------------+--------------+-----------+----------+-----------+---------------------+---------------------+---------------------+--------+
|     82 | test    | t          | alter table multi-schema change | none         |         2 |       80 |         0 | 2023-01-06 20:45:32 | 2023-01-06 20:45:32 | 2023-01-06 20:45:33 | synced |
|     82 | test    | t          | add index /* subjob */          | public       |         2 |       80 |         0 | NULL                | NULL                | NULL                | done   |
|     82 | test    | t          | add column /* subjob */         | public       |         2 |       80 |         0 | NULL                | NULL                | NULL                | done   |
+--------+---------+------------+---------------------------------+--------------+-----------+----------+-----------+---------------------+---------------------+---------------------+--------+