Change the feature flag name reference to const string

[zhiyuanliang-ms]

Currently, our testcases are using Enum.GetName(typeof(Features), Features.TargetingTestFeature) to reference a feature flag.

[zhiyuanliang-ms]

FeatureFlagDemo example project should also be updated.

[juniwang]

Could you explain what's wrong with name reference?

Also, I'm aslo wondering why not just use nameof? For example, nameof(Features.TargetingTestFeature), which sounds pretty simple and straightforward.

[zhiyuanliang-ms]

@juniwang The context is here: #268 #269
Jimmy and I discussed this offline. We prefered to use const string. There was also a PR to update the README: #275