Default values need a protocol

[zachdaniel]

As noted in this issue: ash-project/ash#310

Our default value handling is rudimentary. What we do currently:

defp default(%{default: value, type: type}, _) do
    case Ash.Type.dump_to_native(type, value) do
      {:ok, value} when is_struct(value) ->
        to_string(value)

      {:ok, value} ->
        inspect(value)

      _ ->
        "nil"
    end
  rescue
    _ ->
      "nil"
  end

But what we need to do is add a protocol like AshPostgres.ToSql that can be implemented for various types and structs.