Add New Shipping Method

When selecting shipping_methods while checkout, there would be a process spinned for each listed shipping_method to calculate applicable shipping_cost.

All the shipping_methods which are applicable & respond with in allowed wait time (timeout) will be consolidated, and would be presented in checkout shipping step.

1. Create a module, preferably with new name of Shipping Method, say new_shipping_method and add use Nectar.ShippingCalculator.Base for overridable behavior

   # web/calculators/shipping_calculator/new_shipping_method.ex
   defmodule Nectar.ShippingCalculator.NewShippingMethod do
     use Nectar.ShippingCalculator.Base
   
     # calculate_shipping/1, applicable?/1
     # can also be re-defined 
     def shipping_rate(_order) do
       # Add your shipping_method cost logic
       cost = some_complex_logic
       Decimal.new(cost)
     end
   end

2. Register the new_shipping_method in available shipping_calculators

   config :nectar, :shipping_calculators,
     ...
     new_shipping_method: Nectar.ShippingCalculator.NewShippingMethod,
     ...

3. Add the new_shipping_method in DB %{name: "new_shipping_method", enabled: true}

4. Shipping Methods can be enabled/disabled from Settings

Note: the configuration key should be same as name of added shipping_method in DB or vice-versa.