[MM-29157] Cloud invoices logic (#16056)

* Cloud invoices logic

This commit includes all the code needed to get and print invoices
from CWS

einterfaces/cloud.go

@@ -18,4 +18,6 @@ type CloudInterface interface {
 	UpdateCloudCustomerAddress(address *model.Address) (*model.CloudCustomer, *model.AppError)
 
 	GetSubscription() (*model.Subscription, *model.AppError)
+	GetInvoicesForSubscription() ([]*model.Invoice, *model.AppError)
+	GetInvoicePDF(invoiceID string) ([]byte, *model.AppError)
 }