Add row-level append() API for single-record operations in python bindings

[fresh-borzoni]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Python bindings only support batch operations (write_arrow_batch, write_pandas).
No way to write individual records

Solution

Add append(row) method:

writer.append({'id': 1, 'name': 'Alice'})
writer.append([2, 'Bob'])

Implementation

• Core API already exists: AppendWriter::append(GenericRow)
• Convert Python dict/list → GenericRow
• Use Datum::OwnedString (no memory leak)

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!

[luoyuxia]

@fresh-borzoni Thanks. That will be great. I'm always want to have it.