Added: Support for Pause and Resume

[Sewer56]

Job Pause/Resume Implementation

Spent a bit thinking about the problem; including how to best handle
requested upcoming features such as downloads pausing/resuming.

I think the solution here is decent; it's a bit different than originally proposed, which was a cancel/restart.
This is more of a true pause/resume, by stalling existing jobs without hogging the threadpool.

Summary

Adds pause/resume functionality to the job system using a cooperative model, with both standard and pre-emptive pause modes.

Jobs can be paused using existing YieldAsync() yield points and resumed later,
with cancellation taking precedence over pause.

Main Points

• JobCancellationToken: New token combining cancellation + pause/resume using AsyncManualResetEvent
• YieldAsync(): Now handles both pause detection and waiting for resume
  • Many of our existing jobs are now pausable without requiring code changes.
• API: Added Pause()/Resume() methods to IJob, IJobGroup, IJobMonitor
• Force Pause: For jobs with long-running nested operations a 'pre-emptive' pause mode is available:
  • Set SupportsForcePause => true and handle OperationCanceledException with HandlePauseExceptionAsync()
  • See example.

Tests & Examples

• Example (co-operative) usage in Examples/BestPractices/PauseResumeExample.cs
• Force pause (pre-emptive) example in Examples/BestPractices/ForcePauseExample.cs
• Test for new JobCancellationToken (e.g. Pausing, Waiting, etc.) in NexusMods.Jobs.Tests/Unit/JobCancellationTokenTests.cs
• Test for pausing in jobs in NexusMods.Jobs.Tests/Unit/JobPauseResumeTests.cs
• Test for force pause functionality in NexusMods.Jobs.Tests/Unit/JobForcePauseResumeTests.cs

Misc

• Added AsyncEx package rather than implementing own AsyncManualResetEvent by hand.

Tests were written with extra care to prevent flakiness in CI.
As we know, the Windows GitHub Actions test runner can be unreliable and stall several seconds at a time. So I've been very careful with how we wait.

fixes #3604

[erri120]

This doesn't follow the requirements set out by #3604.

If you think a different approach to #3604 is more appropriate, you should've commented on #3604, explained your approach and allowed for others to comment on it as well.

[Al12rs]

From looking at the code, the main reason this alternative approach was implemented was to:

• Support cancellation of a paused job

But the pausing approach proposed doesn't work well for our purposes because it

• requires cooperative YieldAsync calls rather than Token cancellation, which means that we can't pause external code that takes tokens.
• We want pause to stop operation even if inside nested async calls that supports token cancellation.