csrgen: support initialising OpenSSL adaptor with key object
    
    As a convenience for using it with the test suite, update the csrgen
    OpenSSLAdaptor class to support initialisation with a
    python-cryptography key object, rather than reading the key from a
    file.
    
    Part of: https://pagure.io/freeipa/issue/7496
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>