expressions: make unknown column distinguishable

This patch makes "UnKnownColumn" expression distinguishable from actual
"Column" expression to have an ability to compare them in planning tests.
For now "UnKnownColumn" string representation is started from `?` symbol,
for example, `partitioning=Hash([`?a@0`], 1)`.

Author: Альберт Скальт

datafusion/physical-expr/src/expressions/unknown_column.rs

@@ -51,7 +51,7 @@ impl UnKnownColumn {
 
 impl std::fmt::Display for UnKnownColumn {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        write!(f, "{}", self.name)
+        write!(f, "?{}", self.name)
     }
 }