Make it easier to add design-time services

[ajcvickers]

See dotnet/EntityFramework.Docs#4013

Specifically, not needing to do this manually:

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