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ARROW-7324: [Rust] Add timezone to timestamp

This changes `DataType::TimeStamp(TimeUnit)` to `DataType::TimeStamp(TimeUnit, Option<String>)` where the `Option<String>` is an optional timezone.

I would like some feedback on whether this option is fine. I haven't tried `Option<&str>` to avoid introducing a lifetime on the `DataType`.

The timezone support should practically only affect temporal kernels (`fn value_as_date_time()`), but I haven't done anything there. This could be a follow up PR if there's a wider need for timezones in temporal kernels (I'll likely need them, but not urgent).

Are you fine with the current proposal/implementation @andygrove @paddyhoran @sunchao @liurenjie1024

CC @andy-thomason to keep you in the loop.

Closes #5970 from nevi-me/ARROW-7324 and squashes the following commits:

694bb13fe <Neville Dipale> remove From<Vec<i64>> from timestamp arrays
7d3dba719 <Neville Dipale> test timezone roundtrip on IPC schema conversion
64bcfa3b1 <Neville Dipale> add constructors for timestamps with timezones
6746f9647 <Neville Dipale> ARROW-7324:  Add timezone to timestamp

Authored-by: Neville Dipale <nevilledips@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>