Varaint Type: Add logical type annotation for writer

### Is your feature request related to a problem or challenge?

Follow up to https://github.com/apache/iceberg-rust/pull/2188#discussion_r

iceberg-rust writes via `AsyncArrowWriter`, which derives the Parquet schema from the Arrow schema. In parquet 58.1.0, that path only emits the VARIANT annotation when the field carries the `parquet_variant_compute::VariantType` extension type and variant_experimental is enabled (otherwise logical_type_for_struct is a stub returning None). I couldn't find a public per-field hook to inject the annotation onto a plain `Struct(Binary,Binary)`.

So the real cost is: enable `variant_experimental` + attach the extension type to the field. Two risks that I see:
1. Turning on the feature may change how the reader decodes a VARIANT-annotated group (native VariantArray instead of Struct{metadata,value}) — could break the current read path that expects the struct.
2. New experimental dep surface.

Surfaced here: https://github.com/apache/iceberg-rust/pull/2188#discussion_r3326706646

### Describe the solution you'd like

_No response_

### Willingness to contribute

None

Start with the AsyncArrowWriter path and parquet 58.1.0's logical_type_for_struct behavior, then review the VariantType extension and the discussion in PR #2188. Determine how a per-field VARIANT annotation could be added while preserving the current Struct{metadata,value} read path; done means the risks around variant_experimental and the experimental dependency are resolved.

Written by the indexing model from the issue text.