Review use of IConfiguration/IConfigurationRoot

As per the discussion at corvus-dotnet/Corvus.Identity#13 (comment) we are attempting to move away from using either IConfiguration or IConfigurationRoot in the guts of our IP for the reasons discussed at corvus-dotnet/Corvus.Identity#11

This project currently uses configuration types directly. We need to review whether it runs into the problems described in the links above, and if so, make the relevant changes.

As discussed in the conversation linked to, we have two goals:

- [ ] Avoid any undesirable use of `IConfiguration[Root]`
- [ ] Review the DI setup requirements to see if there are opportunities to simplify normal usage

If any problems or possibly improvements come to light on the second point, please create new work items for them.