[PM-16684] Integrate Pricing Service behind FF (#5276)

* Remove gRPC and convert PricingClient to HttpClient wrapper

* Add PlanType.GetProductTier extension

Many instances of StaticStore use are just to get the ProductTierType of a PlanType, but this can be derived from the PlanType itself without having to fetch the entire plan.

* Remove invocations of the StaticStore in non-Test code

* Deprecate StaticStore entry points

* Run dotnet format

* Matt's feedback

* Run dotnet format

* Rui's feedback

* Run dotnet format

* Replacements since approval

* Run dotnet format

src/Core/Billing/Constants/StripeConstants.cs

@@ -34,6 +34,7 @@ public static class StripeConstants
     public static class InvoiceStatus
     {
         public const string Draft = "draft";
+        public const string Open = "open";
     }
 
     public static class MetadataKeys