Quality of Life improvements for Blob writes

### Task Description

**What needs to be done:**

Let users write a BLOB column without spelling out the unused sibling field.
Today every row needs the full 3-field `{type, data, reference}` struct, even
when the row carries only inline bytes (or only an external reference).

- INLINE writes should accept `{type, data}`.
- OUT_OF_LINE writes should accept `{type, reference}`.
- The missing field gets padded to `null` at the writer entry; canonical
3-field input is still a no-op.
- Padding should also work when the BLOB is nested inside a struct, an array,
or a map.

**Why this task is needed:**

The current `named_struct(... 'reference', cast(null as struct))` boilerplate
(or its DataFrame equivalent) is noisy and easy to get wrong, especially for
INLINE blobs where the reference field has no meaning. It's the first thing
people hit when they try to write a blob, so it shapes the ergonomic
impression of the feature.

### Task Type

Code improvement/refactoring

### Related Issues

**Parent feature issue:** (if applicable )
**Related issues:**
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