Feature Request: Preferred chain

[Alan-Liang]

LE is going to switch the default certificate chain to its own ISRG Root X1 in January 2021, 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.

[plasger-ebv]

Currently, LE offers two Certificate chains, one default and one alternate chain. Would like to have the opportunity to chose chain while generating certificate. Thank you.