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ARROW-9609: [C++][Dataset] CsvFileFormat reads all virtual columns as null

`ConvertOptions::include_missing_columns = true` was insufficient to produce the required behavior with missing columns: we need to read the csv file's header to find the names of columns actually present in the file before instantiating a StreamingReader. Otherwise the StreamingReader will fill absent columns with `null`, which prevents the projector from materializing them correctly later.

Closes #7896 from bkietz/9609-csv-empty-virtual

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>