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

ipaclient/plugins/cert.py

@@ -143,9 +143,6 @@ class cert_request(CertRetrieveOverride):
                 raise errors.CertificateOperationError(
                     error=(_('Generated CSR was empty')))
 
-            # cert_request requires the CSR to be base64-encoded (but PEM
-            # header and footer are not required)
-            csr = unicode(base64.b64encode(csr))
         else:
             if database is not None or private_key is not None:
                 raise errors.MutuallyExclusiveError(reason=_(