feat: include an 'api' option to customize the slack api method url

[zimeg]

Summary

This PR introduces an api option to the inputs of this action as a "custom API URL to send Slack API method requests to".

Resolves #389.

Preview

  - name: Send to a custom API URL
    uses: slackapi/slack-github-action@v2.0.0
    with:
+     api: http://localhost:8080
      method: chat.postMessage
      token: ${{ secrets.SLACK_BOT_TOKEN }}
      payload: |
        channel: ${{ secrets.SLACK_CHANNEL_ID }}
        text: "What's happening on localhost?"

Notes

Changes in this PR also include refactors of #408 needed for testing 🙏 ✨

Super open to naming suggestions since "api" might not be so clear right away, but IMO it matches "method" well 🤔

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (21be76f) to head (2b91f3c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #409   +/-   ##
=======================================
  Coverage   99.85%   99.85%           
=======================================
  Files           7        7           
  Lines         699      702    +3     
=======================================
+ Hits          698      701    +3     
  Misses          1        1           

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[WilliamBergamin]

LGTM

[zimeg]

@WilliamBergamin Thank you for the review! 🙏 ✨

I've confirmed the api option can be used to send requests to development setups, so will go ahead and merge this now 🚢 💨