[AC-1904] Implement endpoint to retrieve Provider subscription (#3921)

* Refactor Core.Billing prior to adding new logic

* Add ProviderBillingQueries.GetSubscriptionData

* Add ProviderBillingController.GetSubscriptionAsync

src/Core/Billing/Models/ConfiguredProviderPlan.cs

@@ -0,0 +1,22 @@
+using Bit.Core.Billing.Entities;
+using Bit.Core.Enums;
+
+namespace Bit.Core.Billing.Models;
+
+public record ConfiguredProviderPlan(
+    Guid Id,
+    Guid ProviderId,
+    PlanType PlanType,
+    int SeatMinimum,
+    int PurchasedSeats)
+{
+    public static ConfiguredProviderPlan From(ProviderPlan providerPlan) =>
+        providerPlan.Configured
+            ? new ConfiguredProviderPlan(
+                providerPlan.Id,
+                providerPlan.ProviderId,
+                providerPlan.PlanType,
+                providerPlan.SeatMinimum.GetValueOrDefault(0),
+                providerPlan.PurchasedSeats.GetValueOrDefault(0))
+            : null;
+}

src/Core/Billing/Models/ProviderSubscriptionData.cs

@@ -0,0 +1,7 @@
+using Stripe;
+
+namespace Bit.Core.Billing.Models;
+
+public record ProviderSubscriptionData(
+    List<ConfiguredProviderPlan> ProviderPlans,
+    Subscription Subscription);