- Startup.ConfigureContainer allows users to configure a 3rd party DI
container in a first class way in the Startup class. 3rd party containers
plug in via IServiceProviderFactory<TContainerBuilder> configured in
IWebHostBuilder.ConfigureServices.
- Added tests
- Remove IStartupLoader and add `IStartup` instead that matches the signature hosting cares about
- Moved `UseStartup` to extension methods
- Move existing logic into `ConventionBasedStartup` class