Feature Request: Preferred chain

[LE is going to switch the default certificate chain to its own ISRG Root X1 in January 2021][otf], but still offers an alternative chain to the DST Root X3. Users might want to use that since the DST Root is more widely trusted. See [certbot instructions here][certbot].

[otf]: https://letsencrypt.org/2020/11/06/own-two-feet.html
[certbot]: https://community.letsencrypt.org/t/certbot-users-preparing-for-the-isrg-root-transition-january-11-2021/138059