Extensibility: making it easy to register additional services on the internal service provider

[divega]

Consider an API that users can invoke in OnConfiguring() to provide additional service implementations of service types for which EF Core (or providers) support multiple instances. This is similar to ReplaceService but without removing existing implementations.

One example of where this could have been useful is adding additional logging providers using the DI-based mechanisms enabled in 2.0 (aspnet/Logging#346) when EF Core manages the ILoggerFactory in the internal service provider.