Add CLI command to run a CronWorkflow on demand

[simster7]

E.g. argo cron run cron-wf-1. This will allow users to run CronWorkflows on demand and out of schedule.

Note: Since the cron CLI currently only manipulates CronWorkflow resources, we may not want to use it to submit a Workflow directly.

Options to implement this:

1. Use argo cron run cron-wf-1. Implement by Geting the CronWorkflow resource, converting it to a Workflow object and calling util.SubmitWorkflow(...) directly from the CLI.
2. Use argo cron run cron-wf-1. Implement by setting a flag in the CronWorkflow resource intended to instruct the cron controller to run the CronWorkflow immediately, after which the flag is reset to its resting state.
3. Use argo submit --from=CronWorkflow/cron-wf-1. Implement by Geting the CronWorkflow resource, converting it to a Workflow object and calling util.SubmitWorkflow(...). Probably least desirable option because it would require adding the CronWorkflowInterface to the root command.

[alexec]

@simster7 I'm putting this in v2.6 so we can prioritize it alongside other issues.