Derive Macro ToCreateModel 

#[derive(ToCreateModel)]
{
    // Attributes available to this derive:
    #[crudcrate]
    #[active_model]
}

=================== ToCreateModel Macro

This macro:

1. Generates a struct named <OriginalName>Create that includes only the fields where #[crudcrate(create_model = false)] is NOT specified (default = true). If a field has an on_create expression, its type becomes Option<…> (with #[serde(default)]) so the user can override that default.
2. Generates an impl From<<OriginalName>Create> for <ActiveModelType>> where:
   • For each field with on_create:
     • If the original type was Option<T>, then create.<field> is Option<Option<T>>. We match on that and do:

       match create.field {
         Some(Some(v)) => Some(v.into()),      // user overrode with T
         Some(None)    => None,                // user explicitly set null
         None          => Some((expr).into()), // fallback to expr
       }

     • If the original type was non‐optional T, then create.<field> is Option<T>. We match on that and do:

       match create.field {
         Some(v) => v.into(),
         None    => (expr).into(),
       }

   • For each field without on_create:
     • If the original type was Option<T>, we do create.<field>.map(|v| v.into()).
     • If it was non‐optional T, we do create.<field>.into().
   • For any field excluded (create_model = false) but having on_create, we do Some((expr).into()) if it was Option<T>, or just (expr).into() otherwise.