feat: Support thread_ts parameter to post the new message as a thread

[ArnaudRinquin]

Closes #187

Summary

Describe the goal of this PR. Mention any related Issue numbers.

Requirements (place an x in each [ ])

• 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.

[WilliamBergamin]

Thanks for the contribution @ArnaudRinquin 💯

Before I move forward with this would you mind testing out what happens if the thread_ts field is passed in the payload?

Based on the behavior from this line it seems like the spread operator should support thread_ts being placed in the payload argument

- name: Send GitHub trigger payload to Slack Workflow Builder
  id: slack
  uses: slackapi/[email protected]
  with:
    channel-id: 'SLACK_CHANNEL_ID' # ID of Slack Channel you want to post to
    payload: |
      {
        "blocks": [
          { "type": "divider" },
          {
            "type": "image",
            "title": {
              "type": "plain_text",
              "text": "Slack Slack Slack",
              "emoji": true
            },
            "image_url": "https://media.makeameme.org/created/a-slack-this.jpg",
            "alt_text": "marg"
          },
          {
            "type": "actions",
            "elements": [
              {
                "type": "button",
                "text": {
                  "type": "plain_text",
                  "text": "${{ github.sha }}"
                },
                "url": "https://github.com/${{ github.repository }}/actions/runs/${{ github.run_id }}"
              }
            ]
          }
        ],
        "thread_ts": "TIMESTAMP_OF_A_MESSAGE"
      }
  env:
    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}

[ArnaudRinquin]

@WilliamBergamin I've added 2 test cases ensuring payload provided thread_ts is still used + overrides input passed thread-ts (I'm not sure which should be used in priority but seems better this way).

[WilliamBergamin]

@ArnaudRinquin these are awesome changes 💯 but implementing the simplest solution for this problem should create a more maintainable project

It would be awesome if this PR could include the unit tests that ensures payload provided thread_ts is valid and documentation on how to use payload provided thread_ts. With this we will be able to update and close #187

[ArnaudRinquin]

@WilliamBergamin

ensures payload provided thread_ts is valid

I'd be happy to validate the thread_ts value but I can't find any spec on the expected shape. Even in official doc the type looks like it can be a simple string. I'll had more tests if you can provide me some hints on how to validate.

I've updated README.md example of specifying thread_ts from payload.

[fleytman]

@WilliamBergamin, what is blocking this pr at the moment?

[WilliamBergamin]

Closing in favor of #309