3.0.0 Breaking changes: rename what sdk calls itself internally

This should only impact anyone using the sdk server side. Clientside users should not (generally) be interacting with the sdk instance

ApiClient => Sdk

Reason is that ordercloud now has a resource for managing ApiClients so there is a name clash. This is a breaking change because it changes how you access the client instance.

var defaultClient = OrderCloud.ApiClient.instance;

becomes:

var defaultClient = OrderCloud.Sdk.instance;