Pass ParsePayload<T>::IS_REQUIRED to T instead of defaulting to …true

[Nahuel-M]

Currently, it's impossible to request or respond with an optional Json body.
Example:

 #[oai(path = "/opa", method = "get")]
    pub async fn get(&self, Json(test): Json<Option<Klaboom>>) -> Json<String> {
        Json("Hello, world!".to_string())
    }

expands to openapi:

"/opa": {
      "get": {
        "requestBody": {
          "content": {
            "application/json; charset=utf-8": {
              "schema": {
                "$ref": "#/components/schemas/Klaboom"
              }
            }
          },
          "required": true
        },
 ...
}

This PR passes the IS_REQUIRED field to the child T, which solves this.