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ARROW-11879 [Rust][DataFusion] Make ExecutionContext::sql return dataframe with optimized plan

I believe we should expect `ExecutionContext::sql` to return an optimized logical plan (with current applying config) rather than a `DataFrame` with an unoptimized plan.
I believe so because

* it is a high level function that should use the current configuration
* it is hard to optimize the logical plan afterwards, as it already returns a dataframe
* many examples, but also DataFusion `repl`  in docs use `ExecutionContext::sql`

The TPC-H benchmarks don't use `ExecutionContext::sql` which is I guess why it was missed before.

FYI @alamb @andygrove

Closes #9639 from Dandandan/ctx_sql_optimize

Authored-by: Heres, Daniel <danielheres@gmail.com>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>