ScalarVariable Expr --> String Support

[alamb]

Is your feature request related to a problem or challenge?

Part of #9726 to complete the Expr

Converting Expr back to SQL is valuable for several usecases such as using DataFusion in federated databases (e.g. generate SQL to push down) or using DataFusion's expr APIs to programatically create SQL.

Describe the solution you'd like

Support converting SQL syntax like this:

@foo

Describe alternatives you've considered

The basic pattern is:

1. Implement the Expr --> AST reverse code in Unparser::expr_to_sql(source link)
2. Add a test to expr_to_sql_ok

Note you can run the tests like

cargo test -p datafusion-sql -- expr_to_sql_ok

Additional context

No response