Fix ignored properties (like payload) on the SlackExecution task

[anna-geller]

Describe the issue

Try running this flow in response to a failed flow -- there are a bunch of properties which are entirely ignored incl:

• payload
• channel - the channel of the webhook url is taken instead
• username - the name of the Slack app is used instead
• iconUrl - the icon of the Slack app is used instead

We should either fix or remove any SlackExecution properties that are currently ignored.

id: sla_miss_alert
namespace: alerting

tasks:
  - id: send_alert
    type: io.kestra.plugin.notifications.slack.SlackExecution
    url: xxxxx
    executionId: "{{trigger.executionId}}"
    iconUrl: "https://avatars.githubusercontent.com/u/59033362?s=48"
    username: "SLA Miss Alert"
    channel: "#random"
    payload: |
      {
        "text": "SLA breached for flow {{trigger.namespace}}.{{trigger.flowId}} with ID {{trigger.executionId}}"
      } 

triggers:
  - id: alert_on_failure
    type: io.kestra.plugin.core.trigger.Flow
    states:
      - FAILED
      - WARNING
      - CANCELLED

Environment

• Kestra Version: develop