Add an option to prefer alternate certificate link
As Let's Encrypt gives two different ways to sign the requested certificates, acme-dns-tiny needs to give an option to choose which one to use.
As reported by Let's Encrypt, in January 2021, they'll serves certificates signed by their new ISR root certificate. Due to the fail of Android updates, only ~66% of devices will be compatible with the ISRG root certificates (Android version less than 7.1 will not be compatible, except if the user uses Firefox to navigate on the web).
At this date, Let's encrypt will give an alternate link to continue to use certificates cross-signed by Iden Trust. To give users a little break up to September 2021, site owner can get certificate from this alternate link instead of the primary link.
So, acme-dns-tiny needs to give an option, like cert-bot, to prefer an alternate link for certificates if available.