Allow for creating structs, with fields of certain type, default value, custom function and more.

Example:

defmodule Sibling do
  use Dry
  alias Dry.Types

  schema do
    attribute(:name, Types.String)
  end
end

defmodule User do
  use Dry
  alias Dry.Types

  schema do
      attribute(:name, Types.String)
      attribute(:age, Types.Integer.options(optional: true))
      attribute(:height)
      attribute(:country, Types.String.options(default: "UK"))
      attribute(:siblings, Types.Array.options(type: Sibling))
      attribute(:favourite_colours, Types.Array.options(type: Types.String, default: ["blue", "green"]))

      attribute :is_adult do
        Map.get(entity, :age, 0) >= 18
      end

      attribute :tall do
        Map.get(entity, :height, 0) >= 180
      end
  end
end

user = User.new!(%{name: "Rob", age: 18, height: 169, country: "BG", siblings: [%{name: "John"}]})
user == %User{
  age: 18,
  country: "BG",
  height: 169,
  is_adult: true,
  name: "Rob",
  tall: false,
  siblings: [%Sibling{name: "John"}],
  favourite_colours: ["blue", "green"]
}
{:ok, _user} = User.new(%{name: "Rob", age: 18, height: 169, country: "BG", siblings: [%{name: "John"}]})  ```