Consider adding some means for the caller to know whether the request was from cache or not

We have recently started using your API, and have a bug such that while we _thought_ we were getting the identity flags from cache we apparently weren't.

_We will fix that bug in our consumer shortly, but I mention only to set the context._

The problem is, metrics wise, I can count on my side how often I call `GetIdentityFlags` but what I _can't_ tell is how many requests hit the cache (hooray - unbilled!) and how many didn't (oh no - billed!). So it is hard for me to setup alerting on my side for excessive usage.

What would be valuable is it `GetIdentityFlags` somehow indicated to the caller _whether this came from cache or not_.

Alas I don't have time to implement it myself, but I am hoping my previous efffort (https://github.com/Flagsmith/flagsmith-dotnet-client/pull/126) may have bought me enough brownie points for you to add it to your roadmap. 😸

Start at the GetIdentityFlags entry point and trace how its cache path is selected. Review the context in pull request 126 and determine what caller-visible cache information is required, including how cached and uncached requests should be distinguished. Done means the API behavior and its tests clearly expose that distinction.

Written by the indexing model from the issue text.