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

* Refactor Core.Billing prior to adding new logic

* Add ProviderBillingQueries.GetSubscriptionData

* Add ProviderBillingController.GetSubscriptionAsync

src/Api/Billing/Models/ProviderSubscriptionDTO.cs

@@ -0,0 +1,47 @@
+using Bit.Core.Billing.Models;
+using Bit.Core.Utilities;
+using Stripe;
+
+namespace Bit.Api.Billing.Models;
+
+public record ProviderSubscriptionDTO(
+    string Status,
+    DateTime CurrentPeriodEndDate,
+    decimal? DiscountPercentage,
+    IEnumerable<ProviderPlanDTO> Plans)
+{
+    private const string _annualCadence = "Annual";
+    private const string _monthlyCadence = "Monthly";
+
+    public static ProviderSubscriptionDTO From(
+        IEnumerable<ConfiguredProviderPlan> providerPlans,
+        Subscription subscription)
+    {
+        var providerPlansDTO = providerPlans
+            .Select(providerPlan =>
+            {
+                var plan = StaticStore.GetPlan(providerPlan.PlanType);
+                var cost = (providerPlan.SeatMinimum + providerPlan.PurchasedSeats) * plan.PasswordManager.SeatPrice;
+                var cadence = plan.IsAnnual ? _annualCadence : _monthlyCadence;
+                return new ProviderPlanDTO(
+                    plan.Name,
+                    providerPlan.SeatMinimum,
+                    providerPlan.PurchasedSeats,
+                    cost,
+                    cadence);
+            });
+
+        return new ProviderSubscriptionDTO(
+            subscription.Status,
+            subscription.CurrentPeriodEnd,
+            subscription.Customer?.Discount?.Coupon?.PercentOff,
+            providerPlansDTO);
+    }
+}
+
+public record ProviderPlanDTO(
+    string PlanName,
+    int SeatMinimum,
+    int PurchasedSeats,
+    decimal Cost,
+    string Cadence);