feat: evaluation v2

[bgiori]

Summary

Implement Evaluation V2

• New evaluation binary with updated interface.
• New /v2/vardata and /v2/flags apis for remote and local evaluation
• Additional info sent in Assignment event.
• New evaluate_v2 and fetch_v2 methods for consuming v2 results.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[tyiuhc]

nit: there seems to be an error in the remote fetch_async_with_retry test case - TypeError: callback_for_async() takes 3 positional arguments but 4 were given