Reveal implementing type and return type in simple UDF implementations

Debug trait is useful for understanding what something is and how it's
configured, especially if the implementation is behind dyn trait.

datafusion/expr/src/expr_fn.rs

@@ -416,9 +416,10 @@ pub struct SimpleScalarUDF {
 
 impl Debug for SimpleScalarUDF {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        f.debug_struct("ScalarUDF")
+        f.debug_struct("SimpleScalarUDF")
             .field("name", &self.name)
             .field("signature", &self.signature)
+            .field("return_type", &self.return_type)
             .field("fun", &"<FUNC>")
             .finish()
     }
@@ -524,9 +525,10 @@ pub struct SimpleAggregateUDF {
 
 impl Debug for SimpleAggregateUDF {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-        f.debug_struct("AggregateUDF")
+        f.debug_struct("SimpleAggregateUDF")
             .field("name", &self.name)
             .field("signature", &self.signature)
+            .field("return_type", &self.return_type)
             .field("fun", &"<FUNC>")
             .finish()
     }