* Enable configuring the default service provider
- Added UseDefaultServiceProvider method
- Made DelegateStartup use the IServiceProviderFactory. One downside
here is that we can't use 3rd party DI containers with the Configure
delegate since it's hardcoded to the the specific Startup type but that's
not a regression.
- Remove IStartupLoader and add `IStartup` instead that matches the signature hosting cares about
- Moved `UseStartup` to extension methods
- Move existing logic into `ConventionBasedStartup` class
- This change removes the indirection and between the IServerLoader and
the IServerFactory. We now add the IServerFactory directly to the DI
container and resolve it from there.
- Moved logic that resolves IServerFactory from an assembly to a static
helper