Behaviour for modules that provide their own JSON Schema.

Every JSONCodec module implements it. Other modules implement it to give codec fields that reference them a schema:

defmodule Money do
  @behaviour JSONCodec.Schema

  @impl true
  def schema, do: %{"type" => "string", "pattern" => "^\\d+\\.\\d{2}$"}
end

Referenced modules that implement neither get an empty schema.