Option to add root cert URLs to config

[lnobach]

URLs from where to retrieve the root certificate for the chain if a cert is requested.
New option for ACME cert provider:

      rootCertUrls: # List of URLs where dns3ld can retrieve the PEM-encoded root certificate in case the ACME service
                    # does not provide it in its chain. If empty, chain is provided as-is. If multiple URLs are given,
                    # they are successively tried, in case the cert is a valid root certificate for the chain it is appended
                    # to the chain. Fails with an error if no cert in the chain is a valid root cert.
        - "https://letsencrypt.org/certs/isrgrootx1.pem"
        - "https://letsencrypt.org/certs/isrg-root-x2.pem"