Incorrect Scalar Timezone Coercion

[tustvold]

Describe the bug

The scalar coercion logic currently incorrectly interprets a timezone without a timezone qualifier as a time since the UTC epoch. It should instead interpret it as relative to the timezone it is being cast to.

To Reproduce

#5914 added the following test (known to be incorrect at the time)

query P
select arrow_cast(timestamp '2000-01-01T00:00:00', 'Timestamp(Nanosecond, Some( "+08:00" ))');
----
2000-01-01T08:00:00+08:00

Expected behavior

The correct result is

2000-01-01T00:00:00+08:00

Additional context

Following #5782 which includes the fix for apache/arrow-rs#1936 the array cast is now correct

create table foo as select '2000-01-01T00:00:00' as ts;
❯ select arrow_cast(ts, 'Timestamp(Nanosecond, Some( "+08:00" ))') from foo;
+---------------------------+
| foo.ts                    |
+---------------------------+
| 2000-01-01T00:00:00+08:00 |
+---------------------------+