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ARROW-12277: [Rust][DataFusion] Implement Sum/Count/Min/Max aggregates for Timestamp(_,_)

# Rationale:
If you try and aggregate (via SUM, for example) a column of a timestamp type, DataFusion generates an error:
```
Coercion from [Timestamp(Nanosecond, None)] to the signature Uniform(1, [Int8, Int16, Int32, Int64, UInt8, UInt16, UInt32, UInt64, Float32, Float64]) failed.
```

For example, from IOx

```
> show columns from t;
+---------------+--------------+------------+-------------+-----------------------------+-------------+
| table_catalog | table_schema | table_name | column_name | data_type                   | is_nullable |
+---------------+--------------+------------+-------------+-----------------------------+-------------+
| datafusion    | public       | t          | a           | Utf8                        | NO          |
| datafusion    | public       | t          | b           | Timestamp(Nanosecond, None) | NO          |
+---------------+--------------+------------+-------------+-----------------------------+-------------+
2 row in set. Query took 0 seconds.
> select sum(b) from t;
Plan("Coercion from [Timestamp(Nanosecond, None)] to the signature Uniform(1, [Int8, Int16, Int32, Int64, UInt8, UInt16, UInt32, UInt64, Float32, Float64]) failed.")
```

# Changes:
Add support for aggregating timestamp types and tests for same

# Notes
Note this is follow on / more fleshing out of the work done in #9773 by @velvia (👋  thanks for adding Timestamps to `ScalarValue`)

Supporting AVG on timestamps is tracked by https://issues.apache.org/jira/browse/ARROW-12318. It is more involved (as currently Avg assumes the output type is always F64), and not important for myuse case at the moment.

Closes #9970 from alamb/alamb/ARROW-12277-aggregate-timestamps

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