Add support for free form additional properties #495

Add support for free form additional properties which are used to define map field value types. This commit adds free form support for `ToSchema` and `IntoParams` with following syntax. ```rust #[derive(ToSchema)] struct Map { #[schema(additional_properties)] map: HashMap<String, String>, } #[derive(IntoParams)] struct Map { #[param(additional_properties)] map: HashMap<String, String>, } ```