ipaldap.py: fix method creating a ldap filter for IPACertificate
    
    ipa user-find --certificate and ipa host-find --certificate
    fail to return matching entries, because the method transforming
    the attribute into a LDAP filter does not properly handle
    IPACertificate objects.
    Directory Server logs show a filter with
    (usercertificate=ipalib.x509.IPACertificate object at 0x7fc0a5575b90>)
    
    When the attribute contains a cryptography.x509.Certificate,
    the method needs to extract the public bytes instead of calling str(value).
    
    Fixes https://pagure.io/freeipa/issue/7770
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>