Add TableChangesScan::schema method to get logical schema (#589)

## What changes are proposed in this pull request?
This PR adds a pub method to fetch the logical schema of a
`TableChangesScan`


## How was this change tested?
Everything compiles.

kernel/src/table_changes/scan.rs

@@ -213,7 +213,14 @@ impl TableChangesScan {
         }
     }
 
-    /// Get the predicate [`Expression`] of the scan.
+    /// Get a shared reference to the [`Schema`] of the table changes scan.
+    ///
+    /// [`Schema`]: crate::schema::Schema
+    pub fn schema(&self) -> &SchemaRef {
+        &self.logical_schema
+    }
+
+    /// Get the predicate [`ExpressionRef`] of the scan.
     fn physical_predicate(&self) -> Option<ExpressionRef> {
         if let PhysicalPredicate::Some(ref predicate, _) = self.physical_predicate {
             Some(predicate.clone())