Add unique getter to ColumnDef (#2401)

* Add unique getter to ColumnDef

* Update src/entity/column.rs

---------

Co-authored-by: Chris Tsang <chris.2y3@outlook.com>

src/entity/column.rs

@@ -403,6 +403,11 @@ impl ColumnDef {
     pub fn is_null(&self) -> bool {
         self.null
     }
+
+    /// Returns true if the column is unique
+    pub fn is_unique(&self) -> bool {
+        self.unique
+    }
 }
 
 struct Text;