[PM-15179] Implement endpoints to add existing organization to CB provider (#5310)

* Implement endpoints to add existing organization to provider

* Run dotnet format

* Support MOE

* Run dotnet format

* Move ProviderClientsController under AC ownership

* Move ProviderClientsControllerTests under AC ownership

* Jared's feedback

src/Core/Billing/Models/AddableOrganization.cs

@@ -0,0 +1,8 @@
+namespace Bit.Core.Billing.Models;
+
+public record AddableOrganization(
+    Guid Id,
+    string Name,
+    string Plan,
+    int Seats,
+    bool Disabled = false);