Adding an option to have an alternate certificate chain

[alibo]

What would you like to be added:
Adding an option to have an alternate certificate chain to mitigate the impact of the recent let's encrypt changes:

As of January 11, 2021, we’re planning to make a change to our API so that ACME clients will, by default, serve a certificate chain that leads to ISRG Root X1. However, it will also be possible to serve an alternate certificate chain for the same certificate that leads to DST Root X3 and offers broader compatibility. This is implemented via the ACME “alternate” link relation. This is supported by Certbot from version 1.6.0 onwards. If you use a different ACME client, please check your client’s documentation to see if the “alternate” link relation is supported.
https://letsencrypt.org/2020/11/06/own-two-feet.html

Why is this needed:

However, this does introduce some compatibility woes. Some software that hasn’t been updated since 2016 (approximately when our root was accepted to many root programs) still doesn’t trust our root certificate, ISRG Root X1. Most notably, this includes versions of Android prior to 7.1.1. That means those older versions of Android will no longer trust certificates issued by Let’s Encrypt.
https://www.androidpolice.com/2020/11/07/many-websites-will-stop-working-on-older-android-versions-in-2021/

There are still many devices that will face issues.

Other tools:

• Cert Manager: https://cert-manager.io/docs/configuration/acme/#use-an-alternative-certificate-chain
• Ansible: https://docs.ansible.com/ansible/latest/collections/community/crypto/acme_certificate_module.html#examples

@tnozicka

[tnozicka]

sounds reasonable, although we will have to wait for golang/go#42437 to update the library

/priority important-soon

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