Add one item to collection field

[loganmzz]

Instead of setting collection field in a single assignement with filled value, let's support adding item one by one. And optionally, avoid initializing collection field.

Collection field must support Extend trait. However, some collection types must be natively supported:

• BTreeMap
• HashMap
• BTreeSet
• BinaryHeap
• LinkedList
• VecDeque
• Vec
• HashSet

#[derive(Builder)]
#[derive(PartialEq,Debug)]
struct Container {
  vec: Vec<String>,
  map: HashMap<String,String>,
}

let built = Container::builder()
  .push_vec("one"),
  .push_vec("two"),
  .push_map("A", "z")
  .push_map("B","y")
  .build();

assert_eq!(
  built,
  Container {
    vec: vec!["one".into(), "two".into()],
    map: HashMap::from([
     ("A".into(), "z".into()),
     ("B".into(), "y".into()),
    ]),
  },
);