Allow users to stop a previously scheduled cancelation of their Zed Pro plan (#15562)

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>

crates/collab/src/db/tables/billing_subscription.rs

@@ -11,6 +11,7 @@ pub struct Model {
     pub billing_customer_id: BillingCustomerId,
     pub stripe_subscription_id: String,
     pub stripe_subscription_status: StripeSubscriptionStatus,
+    pub stripe_cancel_at: Option<DateTime>,
     pub created_at: DateTime,
 }