Add support for sending thread_broadcast

[dstrates]

Description

Would like the ability to send a threaded reply notification to a channel with thread_broadcast.

This could be surfaced as an optional flag to broadcast the threaded message to the channel, as mentioned in #119

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [x] enhancement (feature request)
• [ ] question
• [ ] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

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Bug Report

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[zimeg]

Hey @dstrates and those following along 👋 We don't support a specific input parameter for the reply_broadcast parameter but this can be included in the payload with a message to decide if the threaded message should be broadcast:

- uses: slackapi/slack-github-action@v1
  with:
    channel-id: "C0123456789"
    payload: |
      {
        "reply_broadcast": true,
        "thread_ts": "${{ steps.example.outputs.ts }}",
        "text": "Status update: LGTM!"
      }
  env:
    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}