move ECP cookie handling into library
Right now the HttpFetcher accepts a cookie string to handle the LIGO.ORG kerberos/shib auth. Code in the main.py CLI can fetch a cookie with ecp-cookie-init, and then extract the needed data from the cookie to pass to HttpFetcher. It would be nice to move this functionality into the library itself, so that users of the module have access to it directly as well.
We might want to streamline the http fetching a bit, maybe move to something more modern like python-requests. We probably at least want to have something that can consume the ecp-cookie-init cookie directly, so we don't have to manually extract the data from the cookie.
ecp-cookie-init will likely be the primary AuthProject-supported way to get the auth cookie. So we'll probably need to depend on it, and then call out to it opportunistically to retrieve cookies as needed.