Add response extension

[karpetrosyan]

Add a response extension that indicates whether or not the response was taken from the cache.

Example:

import httpx
import hishel

with httpx.HTTPTransport() as transport:
    with hishel.CacheTransport(transport=transport) as cache_transport:
        request = httpx.Request(
            "GET",
            "https://example.com",
        )

        response = cache_transport.handle_request(request)
        from_cache = response.extensions['from_cache']
        print(from_cache)