[ResponseOps][Slack Blockkit] support YAML and Hjson as input as well as JSON for blockkit body

[pmuellr]

A while back for the Slack connector, we started supporting using Blockkit to create messages, rather than just the usual single Mrkdown formatted string.

Here's an example in JSON, that might be used for a recovered alert:

{
    "blocks": [
        {
            "type": "header",
            "text": {
                "type": "plain_text",
                "text": "🎉 {{rule.name}} Recovered 🎉",
                "emoji": true
            }
        },
        {
            "type": "section",
            "text": {
                "type": "mrkdwn",
                "text": "View rule: <{{rule.url}}|{{rule.name}}>"
            }
        },
        {
            "type": "section",
            "text": {
                "type": "mrkdwn",
                "text": "<{{context.alertDetailsUrl}}|View alert>"
            }
        },
        {
            "type": "section",
            "text": {
                "type": "mrkdwn",
                "text": "Alert UUID: `{{alert.uuid}}`"
            }
        }
    ]
}

Kinda wordy / noisy as all JSON tends to be. We could make this a little easier by supporting YAML and/or Hjson:

yaml:

blocks:
- type: header
  text:
    type: plain_text
    text: "🎉 {{rule.name}} Recovered 🎉"
    emoji: true
- type: section
  text:
    type: mrkdwn
    text: 'View rule: <{{rule.url}}|{{rule.name}}>'
- type: section
  text:
    type: mrkdwn
    text: "<{{context.alertDetailsUrl}}|View alert>"
- type: section
  text:
    type: mrkdwn
    text: 'Alert UUID: `{{alert.uuid}}`'

Hjson:

{
  blocks: [
    {
      type: header
      text: {
        type: plain_text
        text: 🎉 {{rule.name}} Recovered 🎉
        emoji: true
      }
    }
    {
      type: section
      text: {
        type: mrkdwn
        text: View rule: <{{rule.url}}|{{rule.name}}>
      }
    }
    {
      type: section
      text: {
        type: mrkdwn
        text: <{{context.alertDetailsUrl}}|View alert>
      }
    }
    {
      type: section
      text: {
        type: mrkdwn
        text: Alert UUID: `{{alert.uuid}}`
      }
    }
  ]
}

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[pmuellr]

Not sure if there are other connectors, or perhaps rules, that would also benefit from this sort of ease-of-use, but I'd think anything expecting a non-trivial amount of JSON might be a candidate.