Part of #9494

Is your feature request related to a problem or challenge?

Having a good example helps to make features easier to use in DataFusion

In this case the usecase is programmatic construction of SQL using the DataFUsion Expr and LogicalPlan s using the api from #9726

Describe the solution you'd like

1. Add a new examples directory https://github.com/apache/arrow-datafusion/blob/main/datafusion-examples/examples. Potential names plan_to_sql.rs
2. Add an example that shows how to create an expression using the expr_fn api (e.g. col("x") > 5 AND col("b") = 'foo') and convert that to a string

Describe alternatives you've considered

No response

Additional context

I would expect to extend the same example to converting LogicalPlan --> SQL query as well once #8661 is sufficiently far along (it may be already, I just haven't checked)