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

issue 中沒有指出實作檔案或測試。首先找出專案如何選擇或呈現憑證鏈，然後檢視連結的 Let's Encrypt 與 Certbot 指南；完成後應允許使用者在支援的情況下選擇替代的 DST Root X3 鏈。

由索引模型根據 Issue 內容生成。