* Add IStartupConfigureServicesFilter to wrap ConfigureServices.
* Add IStartupConfigureContainerFilter<TContainerBuilder> to wrap
ConfigureContainer.
* Make StartupLoader build a thunk for configure services that
resolves all instances of IStartupConfigureServicesFilter and
IStartupConfigureContainerFilter<TContainerBuilder> and wraps
invocations to ConfigureServices and ConfigureContainer respectively.
* Refactor building the ConfigureServices callback into a private
builder class due to the increased complexity in the process.
- 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
Javier Calvarro Nelson
David Fowler