Add option to disable cookie persistence in Client/AsyncClient

Initially raised as discussion #1533

I've been using httpx's AsyncClient as a web spider, however I have noticed that cookies are automatically persisted and there's no easy way to disable this. My workaround has been to subclass python's http.cookiejar.CookieJar like so:

from http.cookiejar import CookieJar

class NullCookieJar(CookieJar):
    """A CookieJar that rejects all cookies."""

    def extract_cookies(self, *_):
        """For extracting and saving cookies.  This implementation does nothing"""
        pass

    def set_cookie(self, _):
        """Normally for setting a cookie.  This implementation does nothing"""
        pass

Would it be possible to:

1. add an option to disable cookie persistence to Client/AsyncClient or
2. include this implementation of NullCookieJar in httpx as a utility class?

Thanks!