Support BlobType in read path

[QuakeWang]

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Description

This task tracks a scoped BlobType implementation in paimon-rust as part of #227.

• spec: add BlobType

  • add BlobType to the Rust type system
  • support schema serialization/deserialization
  • add basic tests and fixtures

• read: support blob file format

  • dispatch reads by file format and support .blob via a dedicated reader
  • provide minimal read-only compatibility for Java-generated .blob files in the existing data-evolution flow
  • expose Blob values as Arrow binary bytes
  • add read tests, document current limitations, and fix binding compatibility only if needed

Willingness to contribute

• I'm willing to submit a PR!