Function create_child_folder 

pub async fn create_child_folder(
    http: &Client,
    base_url: &str,
    access_token: &str,
    parent_id: &str,
    display_name: &str,
) -> Result<MailFolder, ClientError>

POST /me/mailFolders/{parent_id}/childFolders: create a child folder under parent_id, returning it.