ARROW-9691: [Rust] [DataFusion] Make sql_statement_to_plan method public
ARROW-9691: [Rust] [DataFusion] Make sql_statement_to_plan method public

Closes #7930 from andygrove/ARROW-9691

Authored-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>
Signed-off-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>

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