Standardise GraphQL client configuration

[nirebu]

Currently the GraphQL client uses an interface that's extracted from a production Shopify custom application.

For broader adoption, this library should interface with the official Shopify API specific GraphQL client.

A built in Customer Account API client is not provided by the official Shopify API gem, however the built in Customer Account API Loader should use a similar interface.

[nirebu]

Closed by #44