[VL] Support native Avro scan

### Description

Adds a Velox-native Avro scan operator, significantly reducing serialization overhead.

Production workloads demonstrate **~80% improvement** in scan performance.

Will submit a Draft PR shortly and iterate on the implementation, work toward merging into Gluten and Velox main.

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### Gluten version

None

The issue proposes a Velox-native Avro scan operator but names no files, tests, or entry points. Start by locating existing scan operators in Gluten and Velox, then determine the integration and validation needed for the proposed implementation to be submitted and merged into both projects.

Written by the indexing model from the issue text.