Support conditionally skipping fields

[leostera]

At the moment there is not attribute support for skipping a field, but we'd like to implement it like Serde.rs does:

let is_new () = true

type user = {
  name: string;
  password: string [@serde { skip; } ];
  created_at: string [@serde { skip_if = is_new () } ]
}
[@@deriving serialize, deserialize]

The expected behavior is that the field will be skipped when found via the next_field call and the field visitor that gets generated.