Protocol Engine: Allow a run to pause itself and resume upon user interaction

[mcous]

Overview

As a user, I'd like to be able to write a protocol that pauses that I can then resume when I tell it to, so that I can check samples and other aspects of my run before continuing.

(This is existing functionality in the current protocol run system that we need to support in the Protocol Engine architecture.)

Acceptance Criteria

• A protocol may pause itself (e.g. via ctx.pause(msg="I am paused now"))
• A user may resume the run by issuing a "resume" action to the protocol run

Not in scope / TBD

• A protocol may resume itself
• A protocol may delay itself for a predetermined time
• A user may override the delay and resume immediately by issuing some action to the protocol run

Implementation Notes

• There will be some trickiness here:
  • When does a protocol initiated "pause" command completed?
    1. When the pause is triggered?
    2. When the protocol is resumed?
  • How similar (or dissimilar) is a protocol initiated pause compared to a user-initiated pause?
  • How does the protocol engine signal out that the run has been paused?