datetime: introduce tz in datetime.parse()

There is an option tz in `datetime.parse()`, it was added in
commit 3c403661 ("datetime, lua: date parsing functions").
The option is not documented, and the commit message says that
option `tz` is "Not yet implemented in this commit.".

The patch added tests and a doc request for this option.
The behaviour of the option `tz` is the same as with option
`tzoffset`:
- if timezone was not set in a parsed string then it is set to
  a value specified by `tz`
- if timezone was set in a parsed string then option `tz` is
  ignored

```
tarantool> date.parse("1970-01-01T01:00:00 MSK", { tz = 'Europe/Paris' })
---
- 1970-01-01T01:00:00 MSK
- 23
...

tarantool> date.parse("1970-01-01T01:00:00", { tz = 'Europe/Paris' })
---
- 1970-01-01T01:00:00 Europe/Paris
- 19
...
```

Follows up #6731
Fixes #10420

@TarantoolBot document
Title: Introduce option `tz` in `datetime.parse()`

The option `tz` is added in a function `datetime.parse()`.
The option set timezone to a passed value if it was not set in
a parsed string.

(cherry picked from commit c6bab23a6dc4f819167cbc78eb93859847a389ea)