Testing around and some tweaks to EF DI/options infrastructure

Or: What's on the end of the stick ,Vic?

This is mostly testing making use of the new patterns for service provider and options injection, including:
- Multiple context types
- Multiple providers
- Constructor injection of OptionsContext
- Readining config (indrectly) from IConfiguration or inline

Also, added the ability to auto-configure a provider if it is the only provider regsitered, which also paves the way for auto-configuring from file-based configuration only.