InstanceConfigurationBuilderItem - класс

public class InstanceConfigurationBuilderItem : ConfigurationBuilderItemBase

public class MyLoaderClass
{
    public async ValueTask<Dictionary<string, object?>?> Invoke(
        Dictionary<string, object?> parameters,
        Dictionary<string, object?> storage,
        Dictionary<string, object?> info,
        CancellationToken cancellationToken = default)
    {
        // parameters - json object from ".include" directive
        // storage - current storage before including, same as context.Storage
        // info - additional info, same as context.Info

        if (parameters.TryGet<object>("uri") is string uri)
        {
            // load configuration from uri
            return new Dictionary<string, object?> { ["Result"] = 42L };
        }

        return null;
    }
}

{
  ".include": [
    {
      ".loader.type": "MyNamespace.MyLoaderClass, MyLibrary",
      "uri": "https://my.configuration.server/files/app.json"
    }
  ]
}