Support writing top level VARIANT columns for flink writer

### Feature Description

**What the feature achieves:**
https://github.com/apache/hudi/issues/18513
Support non-nested / top level variant writes

**Why this feature is needed:**

### User Experience

**How users will use this feature:**
- Configuration changes needed
- API changes
- Usage examples

### Hudi RFC Requirements

**RFC PR link:** (if applicable)

**Why RFC is/isn't needed:**
- Does this change public interfaces/APIs? (Yes/No)
- Does this change storage format? (Yes/No)
- Justification:

Read the linked issue #18513 first to understand the required behavior for top-level VARIANT writes. Then locate the Apache Hudi Flink writer entry point and identify how non-nested VARIANT columns are validated; done means the writer supports these columns and the relevant tests or usage examples pass.

Written by the indexing model from the issue text.