Create a suite of specification tests for Configuration

[divega]

Microsoft.Extensions.Configuration's provider model allows third parties to define their own providers by implementing a set of interfaces. But those interfaces are not enough to define exactly what behaviors those implementations should expose.

We can help by creating specification test package that third parties can use as the starting point for testing their own providers, similar to DI and EF Core.

[HaoK]

This is kind of a pain since there's no common data format, I'll send out a PR for what I'm thinking so far, basically we can feed the providers key/value pairs for them to turn into their own data format, but this basically means every provider needs to implement a key/value => their config file format...which is kinda annoying