Add compatible timezone support for date_format expression

## Description

The `date_format` expression was added in PR #3201, but currently only supports UTC timezone with full compatibility. Non-UTC timezones are marked as `Incompatible` and fall back to Spark by default.

This issue tracks adding proper timezone conversion support so that `date_format` can be fully compatible with Spark for all timezones.

## Current Behavior

- UTC timezone: `Compatible()` - runs natively in Comet
- Non-UTC timezones: `Incompatible()` - falls back to Spark by default
- Users can enable non-UTC with `spark.comet.expr.DateFormatClass.allowIncompatible=true` but results may differ from Spark

## Desired Behavior

All timezones should be `Compatible()` and produce results identical to Spark.

## Technical Details

The current implementation uses DataFusion's `to_char` function which formats timestamps without timezone conversion. Spark's `date_format` applies the session timezone when formatting.

Possible approaches:
1. Convert the timestamp to the target timezone before calling `to_char`
2. Use a timezone-aware formatting function if available in DataFusion
3. Implement custom Rust logic to handle timezone conversion

## Related

- PR #3201 - Initial date_format implementation
- Issue #2649 - Similar timezone issue with `date_trunc`

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> **Note:** This issue was generated with AI assistance.