arrow

ARROW-9986: [Rust] allow to_timestamp to parse local times without fractional seconds

As pointed out on @jhorstmann: https://github.com/apache/arrow/pull/8161#issuecomment-691464193

> One (not directly related) issue I noticed while trying this out, is that the local patterns seem to require the millisecond part, while for utc timestamps with "Z" they are optional:

```
> select to_timestamp('2020-09-12T10:30:00') from test limit 1;
ArrowError(ExternalError(General("Error parsing \'2020-09-12T10:30:00\' as timestamp")))

> select to_timestamp('2020-09-12T10:30:00Z') from test limit 1;
+-------------------------------------------+
| totimestamp(Utf8("2020-09-12T10:30:00Z")) |
+-------------------------------------------+
| 1599906600000000000                       |
+-------------------------------------------+
```

This PR allows parsing local timestamp patterns without the fractional seconds and tests for same

Closes #8179 from alamb/alamb/ARROW-9986-fractional-secs

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Andy Grove <andygrove@nvidia.com>