Create a new class Shopify with getClient() method

Here is the initial proposal. We have 3 new services:
shopify.shopify_client - contains a single instance of Shopify\Clients\Rest
shopify.shopify_client_factory used to create instance of Shopify\Clients\Rest
shopify.shopify is our wrapper on top of contrib HTTP client, where we'll keep some helper methods

Usage:

Procedural/static:
$shopify = Shopify::get();

Services:
Inject shopify.shopify service.

It would be hard to fix the tests now, so we'll proceed without test coverage for now.

Automatically closed - issue fixed for 2 weeks with no activity.