Casting existing timestamp to timestamp again strips timezone information

[jeffreyssmith2nd]

Describe the bug

Using the ::timestamp (or CAST()) function strips timezone information.

To Reproduce

DataFusion CLI v40.0.0
> select arrow_typeof(now()) as tz;
+---------------------------------------+
| tz                                    |
+---------------------------------------+
| Timestamp(Nanosecond, Some("+00:00")) |
+---------------------------------------+

> select arrow_typeof(now()::timestamp) as tz;
+-----------------------------+
| tz                          |
+-----------------------------+
| Timestamp(Nanosecond, None) |
+-----------------------------+

> select arrow_typeof((now() AT TIME ZONE 'America/Denver')::timestamp) as tz;
+-----------------------------+
| tz                          |
+-----------------------------+
| Timestamp(Nanosecond, None) |
+-----------------------------+

Or as seen with actual values:

DataFusion CLI v40.0.0
> select now();
+-----------------------------+
| now()                       |
+-----------------------------+
| 2024-08-28T17:08:08.698528Z |
+-----------------------------+

> select (now() AT TIME ZONE 'America/Denver');
+----------------------------------+
| now()                            |
+----------------------------------+
| 2024-08-28T11:08:13.578025-06:00 |
+----------------------------------+

> select now()::timestamp;
+----------------------------+
| now()                      |
+----------------------------+
| 2024-08-28T17:08:17.228985 |
+----------------------------+

> select (now() AT TIME ZONE 'America/Denver')::timestamp;
+----------------------------+
| now()                      |
+----------------------------+
| 2024-08-28T17:08:20.042920 |
+----------------------------+

Expected behavior

I would expect that using the ::timestamp cast on a timestamp would not impact its timezone. This is how Postgres behaves.

postgres@localhost:postgres> select now()::timestamp
+----------------------------+
| now                        |
|----------------------------|
| 2024-08-28 17:05:07.779323 |
+----------------------------+

postgres@localhost:postgres> select (now() at time zone 'America/Denver')::timestamp
+----------------------------+
| timezone                   |
|----------------------------|
| 2024-08-28 11:05:09.275359 |
+----------------------------+

Additional context

No response