cli: migrate from CoprClient APIv1 call for generating webhook secrets
    
    See #1792
    
    As it turns out, we still use APIv1 for the following command
    
        copr new-webhook-secret <owner>/<project>
    
    Since we don't have this feature implemented in APIv3 yet, I would
    prefer not doing so in the pre-release window, and rather exploit the
    fact that we didn't drop APIv1 support from frontend and send the
    request directly.
    
    APIv3 endpoint should be implemented and this command should be
    migrated ASAP after the release.