Expand Index struct to also get the original field name

This is needed in postgres, since we store data as jsonb so struct
fields can't get renamed

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

heromodels-derive/src/lib.rs

@@ -169,6 +169,10 @@ pub fn model(_attr: TokenStream, item: TokenStream) -> TokenStream {
                 fn key() -> &'static str {
                     #index_key
                 }
+
+                fn field_name() -> &'static str {
+                    #name_str
+                }
             }
         };