feat(stepfunctions): Show state machine executions in the Explorer view.

[peter-smith-phd]

Problem

When developing with AWS Step Functions, the current UI allows the user to start a state machine execution via the
context menu in the AWS Explorer view. However, once the execution has started, it's not possible to view the execution
details, or to learn whether the execution succeed, failed, or is still running. Additionally, developers will want to view the final output of the state machine, as well as the execution history steps. These steps are all necessary for them to proactively debug their state machine code.

Solution

This PR adds state machine executions as child nodes underneath the state machine name in the AWS Explorer. When a state machine execution is started, the new execution is shown (in running state). Up to 10 of the most recent executions are shown in the explorer, which should be enough given that developers will be manually running and validating their state machine, rather than running them at scale.

In future PRs, I'd like to add:

1. A "Stop Execution" context menu item to abort a running state machine execution.
2. The ability to see the final result of the execution.
3. The ability to view the entire history of the execution, similar to what's shown in the Step Functions web console.

This current PR is the basis for this future work.

---

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[amazon-inspector-ohio]

⏳ I'm reviewing this pull request for security vulnerabilities and code quality issues. I'll provide an update when I'm done

[github-actions]

• This pull request implements a feat or fix, so it must include a changelog entry (unless the fix is for an unreleased feature). Review the changelog guidelines.
  • Note: beta or "experiment" features that have active users should announce fixes in the changelog.
  • If this is not a feature or fix, use an appropriate type from the title guidelines. For example, telemetry-only changes should use the telemetry type.

[amazon-inspector-ohio]

✅ I finished the code review, and didn't find any security or code quality issues.

[peter-smith-phd]

Hi @zelzhou and @anthonyting, I recently submitted this PR, but then I saw you're also doing some active work on displaying Step Functions executions. I don't think my change overlaps with your change, but I'd love your feedback on this PR.

Hopefully you're willing to accept community contributions, especially from a former colleague :-)