Support nested field in UPDATE

https://github.com/lancedb/lance/blob/main/rust/lance/src/dataset/write/update.rs#L114-L124

// TODO: support nested column references. This is mostly blocked on the
// ability to insert them into the RecordBatch properly.
if column.as_ref().contains('.') {
    return Err(Error::NotSupported {
        source: format!(
            "Nested column references are not yet supported. Referenced: {}",
            column.as_ref(),
        )
        .into(),
        location: location!(),
    });
}

Start in rust/lance/src/dataset/write/update.rs at lines 114-124 and trace how column references are inserted into a RecordBatch. Investigate the noted RecordBatch limitation before changing the nested-reference handling. Done means UPDATE supports nested column references instead of returning the shown NotSupported error.