ARROW-11440: [Rust] [DataFusion] Add method to CsvExec to get CSV schema
ARROW-11440: [Rust] [DataFusion] Add method to CsvExec to get CSV schema

I needed this in Ballista so that I could implement serde for the CsvExec operator.

Closes #9377 from andygrove/csv-exec-get-schema

Authored-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>
Signed-off-by: Andrew Lamb <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@nerdnetworks.org>

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