Is it possible to add Let's Encrypt preferredChain support in v1.7

Hi,
This is a feature request to port back the preferredChain support to v1.7.
I see v1.7 has the lego upgrade to v4.3.1 so the backend support is there.

With the Let's Encrypt DST Root CA X3 Expiration (September 2021) , there will be need for some sites to choose the default chain or the short chain, as described in OpenSSL Client Compatibility Changes for Let’s Encrypt Certificates

Hello @liejuntao001

I highly recommend upgrading to Traefik V2, we do not add new features to V1. Let me know if you have questions related to migration.

Thank you,