Spike: Context Values: Clarify "Environment Name" segments behaviour

[khvn26]

As we introduce the "Environment Name" context value, we are now unsure how /api/v1/flags should behave for segments that only operate on environment names.

Currently, /api/v1/flags simply returns the environment defaults without calling the segments evaluator. Arguably, it should call get_evaluation_result with provided environment context — this is the behaviour we're currently implementing in local evaluation.

[khvn26]

Right now, it makes sense to me to preserve the get environment flags semantics by passing empty segments array to the engine in APIs and SDKs. We can think on identity-less targeting, and everything that it implies, for the V2 SDK API implementation.