Add an example of how to use the SQL parser/unparser API

[alamb]

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)

[edmondop]

@alamb I wasn't able to get a simple sql example to pass, if I convert the Expr to String directly, the literal is wrapped in a type like so.

     Running `/home/edmondo/Development/arrow-datafusion/target/debug/examples/plan_to_sql`
thread 'main' panicked at datafusion-examples/examples/plan_to_sql.rs:45:5:
assertion `left == right` failed
  left: "((\"a\" < 5) OR (\"a\" = 8))"
 right: "a < 5 OR a = 8"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

However, if I use the expr_to_sql, the column names are quoted,

     Running `/home/edmondo/Development/arrow-datafusion/target/debug/examples/plan_to_sql`
thread 'main' panicked at datafusion-examples/examples/plan_to_sql.rs:45:5:
assertion `left == right` failed
  left: "((\"a\" < 5) OR (\"a\" = 8))"
 right: "a < 5 OR a = 8"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

is it the expectation that column names are always quoted?

[alamb]

is it the expectation that column names are always quoted?

I think so. @phillipleblanc added a way to get the expression back without the quotes -- #10198

Perhaps you could show an example of both?