[PM-12601] Add discount to MSP during creation in Admin Portal (#5391)

* Add Provider DiscountId to database and Stripe customer

* Fix tests

* Add missing EF migrations

* Run dotnet format

src/Core/AdminConsole/Entities/Provider/Provider.cs

@@ -35,6 +35,7 @@ public class Provider : ITableObject<Guid>, ISubscriber
     public GatewayType? Gateway { get; set; }
     public string? GatewayCustomerId { get; set; }
     public string? GatewaySubscriptionId { get; set; }
+    public string? DiscountId { get; set; }
 
     public string? BillingEmailAddress() => BillingEmail?.ToLowerInvariant().Trim();