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_core/src/lib.rs

@@ -83,6 +83,11 @@ pub trait Index {
 
     /// The key of this index
     fn key() -> &'static str;
+
+    /// The original field name. This is the same as [Index::key] by default, unless the user
+    /// specified a `(name=...)` value to the index attribute, in which case [Index::key] will be
+    /// set to the specified key.
+    fn field_name() -> &'static str;
 }
 
 /// Base struct that all models should include