Add `revalidated` extension

[karpetrosyan]

Every response will have a revalidated extension that indicates whether the response has been revalidated or not.

Note that a response could have revalidated set to True even when from_cache is set to False. This occurs when the cached entry has been updated and a new entry is downloaded during revalidation.

>>> import hishel
>>> client = hishel.CacheClient()
>>> response = client.get("https://www.example.com/endpoint_that_is_fresh")
>>> response.extensions["revalidated"]
False
>>> response = client.get("https://www.example.com/endpoint_that_is_stale")
>>> response.extensions["revalidated"]
True