Add option to export the full certificate chain

[ieugen]

Some services require the full certificate chain.
https://kanidm.github.io/kanidm/master/preparing_for_your_deployment.html

It would be nice to have this done as part of certificate generation.
certbot from letsencrypt does that OOTB.

A chain of certificates is all the certs in a file.
You can build one with cat and it's quite easy but it's a common thing that IMO would be great to have.

cat my-cert/cert.pem .tinyca/cert.pem > my-cert/chain.pem