Simple data seeding api

[HeyJoel]

It would be good to make it easier to seed initial data via an update command. You can peice this together now using the existing commands, but it would be useful to build up things like pages using a simpler interface that can automatically connect resources e.g.

var imgRef = builder
    .AddImage()
    .FromSource("/path");

builder
   .AddPage({...})
   .WithTemplate("")
   .ForRegion("MyRegion")
   .AddBlock<MyBlockType>({..});

await builder.RunAsync();

In particular this would be useful to make samples initialize data automatically without having to run SQL scripts.