Add a new parameter type, SerialNumber, as a subclass of Str
    
    Transmitting a big integer like a random serial number over
    either xmlrpc or JSON is problematic because they only support
    32-bit integers at best. A random serial number can be as big
    as 128 bits (theoretically 160 but dogtag limits it).
    
    Treat as a string instead. Internally the value can be treated
    as an Integer to conversions to/from hex as needed but for
    transmission purposes handle it as a string.
    
    Fixes: https://pagure.io/freeipa/issue/2016
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Francisco Trivino <ftrivino@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>