Enhancement \ Discussions: Provide Breakpoints and Input Properties while Testing workflows

As a workflow designer I would like the ability to:

1. Pause, Resume, Cancel the workflow execution at certain activities
2. Provide overridable input (for triggers etc.) for specific actives

I have been reviewing the RunCoreAsync method that uses Events to trigger various stages of the workflow and see the `ActivityExecutionResultExecutedHandler` is responsible for feeding the results of the workflow (via SignalR) to the UI.

I am interested in how we can achieve setting breakpoints (pause points) in the workflow execution activities and pause the workflow at that point awaiting a resume \ cancel event from the UI designer.

I am happy to put some effort into this enhancement however wouldn't mind a little guidance as the best way to pause a workflow via events.

Start by reading RunCoreAsync and ActivityExecutionResultExecutedHandler, then trace how workflow activity events and SignalR updates reach the UI. Define the execution and UI event flow for pausing, resuming, cancelling, and supplying overridable activity inputs; done requires an agreed design and corresponding implementation scope.

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