Add support for free form additional properties

[juhaku]

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.

 #[derive(ToSchema)]
 struct Map {
     #[schema(additional_properties)]
     map: HashMap<String, String>,
 }

 #[derive(IntoParams)]
 struct Map {
     #[param(additional_properties)]
     map: HashMap<String, String>,
 }

The above declarations would render in OpenAPI as seen below.

type: object
additionalProperties: true

Fixes #316