[ECS] [request]: Add support for running a scheduled task immediately

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**Tell us about your request**
I'd like to be able to run a scheduled task immediately without affecting its normal schedule.

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
As I am setting up a new scheduled task I often need to run it outside the normal schedule to test different configurations and perhaps also some code changes. The standard schedule is once a day, so I don't want to wait until next day to see the results.

**Are you currently working around this issue?**
I am working around this issue by editing the schedule manually. I set the cron expression to trigger a minute or two in the future. I'd prefer a button next to the scheduled task to invoke the task immediately.

Begin with the ECS scheduled-task request described here and clarify how an immediate invocation should coexist with the normal schedule. Done means an ECS scheduled task can be invoked immediately without changing its recurring schedule; the issue names no repository files or tests, so implementation entry points still need to be located.

Written by the indexing model from the issue text.