Suport unparsing LogicalPlan::Distinct to DISTINCT

[alamb]

Is your feature request related to a problem or challenge?

Part of #9726 to complete the LogialPlan --> SQL conversion

Converting LogicalPlan back to SQL is valuable for several usecases such as using DataFusion to programatically create SQL -- see the plan_to_sql.rs example

Describe the solution you'd like

Support converting SQL like this:

SELECT DISTINCT x FROM foo;

Describe alternatives you've considered

The basic pattern is this (see #10371 for an example):

1. Implement the LogicalPlan --> AST reverse code in Unparser::plan_to_sql(source link)
2. Add a test in roundtrip_statement in sql_integration.rs source link

Note you can run the tests like

cargo test -p datafusion-sql -- roundtrip_statement

Additional context

I think this is a good first issue as the pattern is well established and there are explicit instructions

[yyy1000]

take