ansible_ipa_server: New functions encode_certificate and decode_certificate

The encode_certificate and decode_certificate are needed to encode and
decode a certificate in the way that it can be passed back from a module
and imported back into a usable certificate in another module.

For newer IPA versions the certificate is normally an IPACertificate for
older IPA versions it is simply a bytes array. But in both cases it needs
to be converted not to break Ansible.