Consider adding support for checkout/3 (Ecto 3)

[darrenklein]

As of the initial alpha release with Ecto 3 support, 624bffb, we do not support checkout/3 - checkout/3 is a @callback that was implemented in Ecto 3 for Ecto.Adapter, but it was not critical to making the upgrade from Ecto 2 to Ecto 3 support.

At the moment, we raise in the event that a user calls this method:

@impl Ecto.Adapter
def checkout(_meta, _opts, _fun) do
  # TODO - consider adding support for this?
  raise "#{inspect __MODULE__}.checkout: #{inspect __MODULE__} does not currently support checkout"
end

Is Ecto's concept of checkout something that is conceptually appropriate to adapt for use in DynamoDB? If so, it should be addressed; otherwise, continuing to raise is probably fine.