Support update with blob encoding column

[jackye1995]

from discord

in rust/lance/src/dataset/write/update.rs:269-291:

1. The update operation creates a scanner with default blob handling (BlobsDescriptions)
2. With this default, blob columns (marked with lance-encoding:blob) are read as descriptions (Struct with position, size)
3. The schema validation at line 286 compares the stream schema (with Struct) against the dataset schema (with LargeBinary)
4. This causes a schema mismatch error