[Rust] JSON writer does not support timestamp types

Looks like the json writer.rs code in arrow doesn't support writing out timestamps. When I tried to write out 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
```