Add Paddle Billing and Stripe sync methods

[excid3]

The goal of this PR is to introduce a simple interface for syncing one-time or subscription transactions. For Stripe, this is syncing Checkout Sessions and determining the Charge or Subscription to sync. For Paddle Billing, this retrieves a Transaction and determines if it is a charge or subscription.

We can also take this a step further and provide a method that takes a hash (like params) and determines whether to sync from Stripe, Paddle Billing, etc.

The end goal is to provide integrations a single method to call for syncing after checkout.

class Checkout::CompletedController < ApplicationController
  def show
    charge_or_subscription = Pay.sync(params)
    # handle fulfillment
    redirect_to root_path, notice: "Thanks for your purchase!"
  end
end