Add support for Workflow Steps from Apps

[misscoded]

Summary

This pull request contains the proposed API changes outlined here to incorporate support for Workflow Steps from Apps.

---

Workflow Steps provide the ability for Slack apps to create and process custom Workflow Steps for use in Workflows. Steps can be built to do things such as send data to external services, create tasks in project management systems, or update tickets. These steps can then be shared and used by anyone in the Workflow Builder to incorporate into Workflows they're building.

For more information on Workflow Steps, read full details here: https://api.slack.com/workflows/steps

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[seratch]

+1

[selfcontained]

outputs should default to an empty object, as it's not an array, but a key/value mapping in the step execute portion.

[misscoded]

Going off of the other comments around config, since outputs is optional per the API docs, I altered this to be the following:

return (config: Parameters<StepCompleteFn>[0] = {}) => {
  return client.workflows.stepCompleted({
    token,
    workflow_step_execute_id,
    ...config,
  });
};

Testing didn't reveal any issues in my case, but let me know if this doesn't suit for some reason!

[misscoded]

Reminder :: @slack/types and @slack/web-api need to be updated to appropriate versions prior to merge

[codecov]

Codecov Report

Merging #597 into feat-workflow-steps will decrease coverage by 2.32%.
The diff coverage is 66.31%.

@@                   Coverage Diff                   @@
##           feat-workflow-steps     #597      +/-   ##
=======================================================
- Coverage                83.27%   80.95%   -2.33%     
=======================================================
  Files                        7        8       +1     
  Lines                      598      693      +95     
  Branches                   193      206      +13     
=======================================================
+ Hits                       498      561      +63     
- Misses                      67       95      +28     
- Partials                    33       37       +4     

Impacted Files	Coverage Δ
src/App.ts	86.77% <0.00%> (-1.09%)	⬇️
src/WorkflowStep.ts	67.41% <67.41%> (ø)
src/errors.ts	100.00% <100.00%> (ø)

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