[Fix] Adapt Arrow timestamp conversion for DATETIME and TIMESTAMPTZ

[JNSimba]

Proposed changes

Background:

PR apache/doris#38215 added timezone support to datetime. Versions prior to this PR hardcoded the use of TimeStampMicroVector, whereas subsequent versions return TimeStampTZVector. Consequently, to ensure compatibility, the connector inferred the time unit based on the timestamp itself.

Changes:

Since datetime is inherently timezone-agnostic, PR apache/doris#65823 removed the timezone from datetime, causing it to return a native timestamp; timestamptz continues to return TimeStampTZVector.

Connector Adaptation:

No conversion is performed if there is no timezone; the existing default conversion logic is applied if a timezone is present.

Impact:

Reading datetime data may result in errors on versions prior to PR apache/doris#38215.

Checklist(Required)

1. Does it affect the original behavior: Yes
2. Has unit tests been added: Yes
3. Has document been added or modified: No Need
4. Does it need to update dependencies: No
5. Are there any changes that cannot be rolled back: No

Further comments

None.