Pretty print metadata in debug persisted CLI

[jopemachine]

After #151, Log entries are now pretty-printed in table format.
However, the metadata is still being output in raw.

I'm not sure if it's appropriate to present the metadata in table format, but we can improve the readability compared to the current state.

For example,

+-----------+-----------------------------------+----------------+
| Category  | Field                             | Value          |
+-----------+-----------------------------------+----------------+
| HardState | term                              | 1              |
|           | vote                              | 2              |
|           | commit                            | 1              |
+-----------+-----------------------------------+----------------+
| ConfState | voters                            | [2, 3, 1]      |
|           | learners                          | []             |
|           | voters_outgoing                   | []             |
|           | learners_next                     | []             |
|           | auto_leave                        | false          |
+-----------+-----------------------------------+----------------+
| Snapshot  | data                              | []             |
|           | metadata                          |                |
|           | ├─ conf_state                     |                |
|           | │  ├─ voters                      | [2, 3, 1]      |
|           | │  ├─ learners                    | []             |
|           | │  ├─ voters_outgoing             | []             |
|           | │  ├─ learners_next               | []             |
|           | │  └─ auto_leave                  | false          |
|           | ├─ index                          | 0              |
|           | └─ term                           | 0              |
+-----------+-----------------------------------+----------------+
| LastIndex | last index                        | 1              |
+-----------+-----------------------------------+----------------+