Give the Optimizely client an interface to facilitate testing

[Nevett]

It is very difficult to write unit tests around our code that makes use of Optimizely, since it provides no interface from which to create a suitable mock. We can make do by creating our own wrapping class, but this gets messy quickly.

Can an IOptimizely interface be added, so that we can use NSubstitute or a similar library to mock the Optimizely client?

[mikeproeng37]

I've closed the PR, but will hold off on closing this until we release. We don't have a date yet, but tentatively within the next few weeks.

[mikeproeng37]

Released in https://github.com/optimizely/csharp-sdk/releases/tag/v2.2.0