Add input_format_csv_allow_variable_number_of_columns option

Why it matters

This feature addresses the problem of importing CSV data where rows may have different numbers of columns. It provides flexibility during data ingestion by allowing ClickHouse to handle CSV files with varying column counts without errors.

How to use it

To enable this feature, set the input_format_csv_allow_variable_number_of_columns option to true when importing CSV data. This can be done by specifying it in the client settings or query settings before reading the CSV input.