Update get one document for v0.28.0

[bidoubiwa]

Add fields parameter in the get_document route.

Breaking

• on the method Index.get_document you are now able to add a field on the fields you want to be returned in the documents.

    #[derive(Serialize, Debug)]
    struct Movie {
       name: String,
       description: String,
       age: Option<usize>
    }
    
   movies.add_or_replace(&[Movie{name:String::from("Interstellar"), description:String::from("Interstellar chronicles the adventures."), age: Some(1)}], Some("name")).await.unwrap().wait_for_completion(&client, None, None).await.unwrap();

    #[derive(Deserialize, Debug)]
    # #[derive(PartialEq)]
    struct ReturnedMovie {
       name: String,
       description: String
    }
    let interstellar = movies.get_document::<ReturnedMovie>("Interstellar", Some(["name", "description"].to_vec())).await.unwrap();
    assert_eq!(interstellar, ReturnedMovie {
      name: String::from("Interstellar"),
      description: String::from("Interstellar chronicles the adventures."),
    });