[UI] Allow user iteration on Workflow plan after pause

Problem

With #109 (closed). Duo Workflow will have the ability to pause before continuing to execution.

Desired Outcome

Users can change the plan as long as they want and duo workflow only goes into execution once they're satisfied with the plan.

Proposed Solution

See video walkthrough:

• Show distinct message to prompt user to review plan when Duo workflow is paused after planning
• Allow user to send message to change plan or "Start" button to start execution.
• Sending message would create another iteration over the plan (implemented as part of #109 (comment 2319940762))

Implementation details

TBD