Support updating numRows in dataframe

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It can be useful for iceberg.

Also, in https://github.com/source-cooperative/csv-table/, I'm parsing a CSV. I want to start by fetching only the first 4MB (for example) to show the first rows, then fetch more data if needed.

We need to know the number of rows to be able to compute the scrollbar size. As we cannot easily know the exact number of rows, we could use an estimate (total_byte_length * init_num_rows / init_byte_length) for that purpose.

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Related feature request, not sure if we really need it though: as CSV can be parsed from the end too, in case we don't have parsed the whole file, but want to inspect the end of the file, we could allow .slice(-20,-1) in AsyncBuffer and adapt HighTable (Ctrl + PgDown -> show the last rows, even if we don't know how many rows the file contains)