Add indexed_fields method on Model

For postgres indexes we want to know the indexed field names upfront
without an actual instance of Model

Signed-off-by: Lee Smet <lee.smet@hotmail.com>

heromodels_core/src/lib.rs

@@ -58,6 +58,11 @@ pub trait Model:
         Vec::new()
     }
 
+    /// Return a list of field names which have an index applied.
+    fn indexed_fields() -> Vec<&'static str> {
+        Vec::new()
+    }
+
     /// Get the unique ID for this model
     fn get_id(&self) -> u32;