[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/Controllers/ProviderBillingController.cs

@@ -0,0 +1,44 @@
+using Bit.Api.Billing.Models;
+using Bit.Core;
+using Bit.Core.Billing.Queries;
+using Bit.Core.Context;
+using Bit.Core.Services;
+using Microsoft.AspNetCore.Authorization;
+using Microsoft.AspNetCore.Mvc;
+
+namespace Bit.Api.Billing.Controllers;
+
+[Route("providers/{providerId:guid}/billing")]
+[Authorize("Application")]
+public class ProviderBillingController(
+    ICurrentContext currentContext,
+    IFeatureService featureService,
+    IProviderBillingQueries providerBillingQueries) : Controller
+{
+    [HttpGet("subscription")]
+    public async Task<IResult> GetSubscriptionAsync([FromRoute] Guid providerId)
+    {
+        if (!featureService.IsEnabled(FeatureFlagKeys.EnableConsolidatedBilling))
+        {
+            return TypedResults.NotFound();
+        }
+
+        if (!currentContext.ProviderProviderAdmin(providerId))
+        {
+            return TypedResults.Unauthorized();
+        }
+
+        var subscriptionData = await providerBillingQueries.GetSubscriptionData(providerId);
+
+        if (subscriptionData == null)
+        {
+            return TypedResults.NotFound();
+        }
+
+        var (providerPlans, subscription) = subscriptionData;
+
+        var providerSubscriptionDTO = ProviderSubscriptionDTO.From(providerPlans, subscription);
+
+        return TypedResults.Ok(providerSubscriptionDTO);
+    }
+}