Customize user-agent of odcs-client
    
    Previouly it uses the default user-agent of requests,
    e.g. "python-requests/2.26.0".
    
    With this patch, it will like "odcs-client/0.3.8 Python/3.6.8".
    
    Signed-off-by: Haibo Lin <hlin@redhat.com>