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ARROW-12267: [Rust] Implement support for timestamps in JSON writer

# Rationale
I want to write timestamps to JSON (e.g. https://github.com/influxdata/influxdb_iox/pull/1104) and when I did so I got a panic :)

When I tried to write a`TimestampNanosecondArray` I got the following error:

```
thread 'influxdb_ioxd::http::tests::test_query_json' panicked at 'Unsupported datatype: Timestamp(
    Nanosecond,
    None,
)', /Users/alamb/.cargo/git/checkouts/arrow-3a9cfebb6b7b2bdc/3e825a7/rust/arrow/src/json/writer.rs:326:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

# Changes
Add support for `DataType::Timestamp`, and tests for same

I did not add support for the other date / time / duration types, but instead filed a ticket to add such support: https://issues.apache.org/jira/browse/ARROW-12317

Closes #9968 from alamb/ARROW-12267/json_writer

Authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>