Improving DI configuration

[Turnerj]

I always thought the DI story was a little janky, requiring you to specify arrays for the layers and extensions. Plus when it comes to configuring any layers, it makes a bit of a mess.

This PR changes that and makes a more fluent API around building a cache stack. Here is the general plan:

Before

services.AddCacheStack<MyContext>(
    new ICacheLayer[] { new MemoryCacheLayer() }, 
    new ICacheExtension[] { new AutoCleanupExtension(TimeSpan.FromSeconds(30)) }
);

After

services.AddCacheStack<MyContext>(builder => builder
    .AddMemoryCacheLayer()
    .WithCleanupFrequency(TimeSpan.FromSeconds(30))
);

The result of these changes should make adding cache layers and extensions easier/more discoverable. Additionally, configuring the cache layers and extensions becomes a bit cleaner.