parameters: introduce CertificateSigningRequest
    
    Previously, CSRs were handled as a Str parameter which brought
    trouble to Python 3 because of its more strict type requirements.
    We introduce a CertificateSigningRequest parameter which allows to
    use python-cryptography x509.CertificateSigningRequest to represent
    CSRs in the framework.
    
    https://pagure.io/freeipa/issue/7131
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>