feat: Add Authenticated Datafile Support

[msohailhussain]

Summary

• Adds support for Authenticated Datafiles.
• Added a HttpClient wrapper for HttpProjectConfigManager.

  Test plan

  Added new unit tests FSC is passing here. https://travis-ci.com/github/optimizely/fullstack-sdk-compatibility-suite/builds/171678195

[msohailhussain]

https://github.com/optimizely/csharp-testapp/pull/65 Please merge this PR first.