feat: table descriptions in _pico_table

Summary

• System table _pico_table format is changed
• New field description is added (string)

Example output:

picodata> select * from "_pico_table"
+-----+----------------------+-----------------+---------------------+----------------+----------+---------+-------+---------------------+
| id  | name                 | distribution    | format              | schema_version | operable | engine  | owner | description         |
+========================================================================================================================================+
| 512 | "_pico_table"        | {"Global": nil} | [{"field_type":     | 0              | true     | "memtx" | 1     | "Stores metadata of |
|     |                      |                 | "unsigned", "name": |                |          |         |       | all the cluster     |
|     |                      |                 | "id",               |                |          |         |       | tables in           |
|     |                      |                 | "is_nullable":      |                |          |         |       | picodata."          |
|     |                      |                 | false},             |                |          |         |       |                     |
|     |                      |                 | {"field_type":      |                |          |         |       |                     |
| ... | ...                  | ...             | ...                 | ...            | ...      | ...     | ...   | ...                 |

• feat: table descriptions in _pico_table

---

• Part of #323
• Cherry-pick to: none
• Docs follow-up: docs#289 (closed)