Hooks feature - Githubissues

Adds ClientReady, FlagEvaluated, ConfigChanged, Error and BeforeClientDispose events to ConfigCatClientsOptions/ConfigCatClients.

Also improves/fixes minor issues of the previously submitted PRs:

To be able to meet the requirement of raising the ClientReady event when "maxInitWaitTimeSeconds passed" in the case of auto polling, I needed to change the ManualResetEventSlim-based approach to initialization handling to a CancellationTokenSource-based one because CTS supports active and timed notifications via RegisterCallback+CancelAfter.
Logging around value evaluation (RolloutEvaluator) has been also revised and cleaned up because the original implementation didn't play nicely with the FlagEvaluated event and mass-evaluation like GetAllValues and GetAllVariationIds.
[x] I have covered the applied changes with automated tests.
[x] I have executed the full automated test set against my changes.
[x] I have validated my changes against all supported platform versions.
[x] I have read and accepted the contribution agreement.

configcat / .net-sdk