ADMIN SHOW DDL has structured data in Go format

## Enhancement

```
mysql> admin show ddl\G
*************************** 1. row ***************************
SCHEMA_VER: 5055
OWNER_ID: eef31ae6-14a2-4664-85d7-dc842ccd62e4
OWNER_ADDRESS: 192.168.1.229:4000
RUNNING_JOBS: ID:10106, Type:add index, State:running, SchemaState:write reorganization, SchemaID:2, TableID:10104, RowCount:1447630, ArgLen:0, start time: 2024-04-26 11:20:02.291 +0200 CEST, Err:, ErrCount:0, SnapshotVersion:449347112749760513, LocalMode: false, UniqueWarnings:0
SELF_ID: eef31ae6-14a2-4664-85d7-dc842ccd62e4
QUERY: alter table t1 add index (c1(100))
1 row in set (0.00 sec)
```

The information in the `RUNNING_JOBS` column should be made into something like JSON that is easier to work with.

Start by locating the implementation of the ADMIN SHOW DDL command and how its RUNNING_JOBS column is formatted. Change that structured job information to a JSON representation, then verify the command output is easier to parse while retaining the existing job details shown in the example.

Written by the indexing model from the issue text.