Make it easier to add design-time services

See https://github.com/dotnet/EntityFramework.Docs/issues/4013

Specifically, not needing to do this manually:

```C#
var provider = context.GetService().Name;
var providerAssembly = Assembly.Load(new AssemblyName(provider));
var providerServicesAttribute = providerAssembly.GetCustomAttribute();
var designTimeServicesType = providerAssembly.GetType(providerServicesAttribute.TypeName, throwOnError: true);
((IDesignTimeServices)Activator.CreateInstance(designTimeServicesType)!).ConfigureDesignTimeServices(serviceCollection);
```