feat(blob): add BlobType groundwork and preserve DDL semantics

[QuakeWang]

Purpose

This PR adds the first groundwork slice for BlobType in paimon-rust.

It introduces BlobType into the Rust type system, preserves BLOB in DataFusion DDL, and explicitly rejects unsupported write paths.

This PR is only a groundwork step for #228, and I will implement .blob read path for the next pr.

Brief change log

• add DataType::Blob and BlobType
• support BlobType schema serialization / deserialization and add serde fixtures
• expose BlobType as Arrow Binary in the existing type conversion surface
• preserve BLOB in DataFusion DDL schema translation, instead of silently degrading it to VARBINARY
• add tests for CREATE TABLE / ALTER TABLE ... ADD COLUMN ... BLOB
• explicitly reject BlobType in current write paths
• explicitly keep BlobType unsupported for partition keys and parquet row filter literals

Tests

API and Format

API change:

• add a new logical type BlobType / DataType::Blob

Format impact:

• schema serde now supports BLOB
• no data file format change in this PR
• .blob file read support is not included in this PR

Documentation

[JingsongLi]

If we have this, we should remove sql_data_type_to_arrow.

[QuakeWang]

@JingsongLi Thanks for the review. sql_data_type_to_arrow has been removed, and DDL type translation now goes directly through sql_data_type_to_paimon_type.

[JingsongLi]

+1