`datetime` module has a function `:totable()` that converts
the information from a datetime object into the table format.
The commit 43e10ed3 ("build, lua: built-in module datetime")
added `tzoffset` field to the datetime object and to table
produced by `:totable()`. The commit 9ee45289
("datetime: datetime.TZ array") added fields `tz` and `tzindex` to
the datetime object, but not to the table produced by
`:totable()`. The patch fixes that. Note, `tzindex` is not added,
because it is an internal field.

```
tarantool> datetime.parse('2004-12-01T00:00 Europe/Moscow'):totable()
---
- tz: Europe/Moscow
  sec: 0
  min: 0
  yday: 336
  day: 1
  nsec: 0
  isdst: false
  wday: 4
  tzoffset: 180
  month: 12
  year: 2004
  hour: 0
...
```

Fixes #10331
Follows up #6751

@TarantoolBot document
Title: Support of tz field in :totable()

In addition to the `tzoffset` in a table produced by `:totable`
we added `tz` field.

```
tarantool> datetime.parse('2004-12-01T00:00 Europe/Moscow'):totable()
---
- tz: Europe/Moscow
  sec: 0
  min: 0
  yday: 336
  day: 1
  nsec: 0
  isdst: false
  wday: 4
  tzoffset: 180
  month: 12
  year: 2004
  hour: 0
...
```

(cherry picked from commit 90552e55e0921405c43ea086ae418a72c9f000e4)