Enhance TimeStampArray to support IANA/Olson TZ strings

### Describe the enhancement requested

# Summary

The format of a Timestamp may include an IANA/Olson timezone identifier, but this is not currently supported by the C# implementation in `TimestampArray`.

From the schema:

```

The time zone is a string indicating the name of a time zone, one of:

* As used in the Olson time zone database (the "tz database" or
"tzdata"), such as "America/New_York"
* An absolute time zone offset of the form +XX:XX or -XX:XX, such as +07:30
```

# Acceptance Criteria

- `TimestampArray` _must_ support IANA/Olson timezone identifiers.

**Reporter**: [Adam Szmigin](https://issues.apache.org/jira/browse/ARROW-9515) / @mr-smidge

_{**Note**: *This issue was originally created as [ARROW-9515](https://issues.apache.org/jira/browse/ARROW-9515). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Copied from https://github.com/apache/arrow/issues/25583

Start by locating the C# implementation of TimestampArray and its existing timezone handling and tests. Compare the current behavior with the schema's IANA/Olson identifier examples, then verify that TimestampArray accepts identifiers such as America/New_York while retaining support for absolute offsets.

Written by the indexing model from the issue text.