Stock availability checker service implements Commerce AvailabilityCheckerInterface

Problem/Motivation

Interface with the Commerce AvailabilityManager service that implements AvailabilityManagerInterface

<?php
  commerce.availability_manager:
    class: Drupal\commerce\AvailabilityManager
    tags:
      - { name: service_collector, tag: commerce.availability_checker, call: addChecker }
?>

This service allows adding instances of AvailabilityCheckerInterface using addChecker.
The service then provides a method to check availability check(PurchasableEntityInterface $entity, $quantity = 1)

Proposed resolution

StockAvailabilityChecker - implements AvailabilityCheckerInterface so can be added to the AvailabilityManager however we have the following points to consider:

1) The StockAvailabilityChecker itself has dependency injection for the EntityStockCheckInterface

2) Should we override the AvailabilityManager to make sure only stock can check if installed? - probably not.

3) The plan is to support more then one instance of EntityStockCheckInterface (probably configured by product type).
We have two options:
a) add one checker - StockAvailabilityChecker acts as a factory for EntityStockCheckInterface determined by the type of the PurchasableEntityInterface passed in.
b) create a AvailabilityCheckerInterface instance for each EntityStockCheckInterface and add those to the AvailabilityManager. Each of those will need to check the PurchasableEntityInterface to find out if it needs to run its own check.

Remaining tasks

• implement StockAvailabilityChecker.
• create stock services interface & implement AlwaysInStockService.
• implement Commerce Stock Storage sub module.
• implement the availability_manager in commerce
• UI and code to determine the stock services tot use