Support response output

[SnowCait]

Description

Action outputs API response, we can get thread_ts value in next steps.

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

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

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

• 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.
• I've searched for any related issues and avoided creating a duplicate issue.

---

Example

jobs:
  post:
    runs-on: ubuntu-latest
    env:
      SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}

    steps:
      - uses: slackapi/slack-github-action@v1.18.0
        id: slack
        with:
          channel-id: '#general'
          slack-message: "Summary"
      - uses: slackapi/slack-github-action@v1.18.0
        with:
          channel-id: '#general'
          payload: |
            {
              "text": "Details",
              "thread_ts": "${{ fromJSON(steps.slack.outputs.response).ts }}"
            }

[stevengill]

Hmm this is a pretty good idea.

I don't think we would be able to provide this for the webhook usecases, but would be useful when using the bot token route.

example output from a successful chat.postMessage

{
    "ok": true,
    "channel": "C1H9RESGL",
    "ts": "1503435956.000247",
    "message": {
        "text": "Here's a message for you",
        "username": "ecto1",
        "bot_id": "B19LU7CSY",
        "attachments": [
            {
                "text": "This is an attachment",
                "id": 1,
                "fallback": "This is an attachment's fallback"
            }
        ],
        "type": "message",
        "subtype": "bot_message",
        "ts": "1503435956.000247"
    }
}