Flatten the generated zip archives and include individual certs

[zacikpa]

Currently, we export each chain as a single file chain.pem together with the root cert root.pem. This makes it hard to validate manually using command-line OpenSSL, since it only accepts a single cert per file.

This PR flattens the archive structure (no subdirectories as is the case now) and also exports each certificate separately.

An example archive may look like this:

• EXPIRED
  • chain.pem (contains endpoint and two intermediate CA certs)
  • root.pem
  • endpoint.pem
  • intermediate1.pem
  • intermediate2.pem

Solves #104.