datetime: handle timezone names in tz

Since recently we partially support timezone names (i.e. as
abbreviations) so we may modify tz attribute support for
datetime constructors or :set() operations.

Closes #7076
Relates to #7007

@TarantoolBot document
Title: datetime tz attribute

Now `tz` attribute is properly handled in datetime value
constructors or `:set{}` method modifiers.

```
tarantool> T = date.new{year = 1980, tz = 'MSK'}
---
...

tarantool> T.tzoffset
---
- 180
...

tarantool> T.tz
---
- MSK
...
tarantool> T = date.new{year = 1980, tzoffset = 180}
---
...

tarantool> T.tzindex
---
- 0
...

tarantool> T.tz
---
-
...

tarantool> T.tzoffset
---
- 180
...

tarantool> T:set{tz = 'MSK'}
---
...

tarantool> T.tz
---
- MSK
...

```