Service interfaces

[nozzlegear]

This pull request will add interfaces for all ShopifySharp service classes.

TODO:

• Update contribution docs (all new services should have an interface; all new methods should be added to existing interface).
• Determine how to handle the HttpClient/HttpClientFactory stuff on the ShopifyService/IShopifyService class and interface. I'm okay with leaving this off for now and implementing in the next pull request, but want to start considering it.
  • Should the instance-specific service.SetHttpClient be replaced with a service.SetHttpClientFactory instead? See my todo here.
  • Or should a constructor overload be added that accepts an IHttpClientFactory? This could make the services more DI friendly (I want to implement a ShopifyServiceCredentials class or something like it so we don't have to pass strings to the services).