[MM-47566] Add upcominginvoice field to Subscription type (#21717)

* Add upcominginvoice field to Subscription type

* Add support for fetching upcoming invoices

* Put back upcominginvoice

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

model/cloud.go

@@ -16,6 +16,8 @@ const (
 	EventTypeTriggerDelinquencyEmail      = "trigger-delinquency-email"
 )
 
+const UpcomingInvoice = "upcoming"
+
 var MockCWS string
 
 type BillingScheme string
@@ -152,6 +154,7 @@ type Subscription struct {
 	DNS                     string   `json:"dns"`
 	IsPaidTier              string   `json:"is_paid_tier"`
 	LastInvoice             *Invoice `json:"last_invoice"`
+	UpcomingInvoice         *Invoice `json:"upcoming_invoice"`
 	IsFreeTrial             string   `json:"is_free_trial"`
 	TrialEndAt              int64    `json:"trial_end_at"`
 	DelinquentSince         *int64   `json:"delinquent_since"`