Add support for `gocd.guest.user.auth.plugin`

[grundic]

Right now it's possible to login with guest like this:

from yagocd import Yagocd

go = Yagocd(server='https://build.gocd.io')

# login as guest
go._session.get('https://build.gocd.io/go/plugin/interact/gocd.guest.user.auth.plugin/index')

print(len(list(go.pipelines)))

Should be separate method, somewhere in Session and special parameter for controlling this.