Support setting body alert at the endpoint level

[pablomendezroyo]

Describe the feature request

Right now, seems like its only supported defining the body of the alert under alerting and not under the endpoint level. e.g:

alerting:
  custom:
    url: "https://hooks.slack.com/services/**********/**********/**********"
    method: "POST"
    body: |
      {
        "text": "[ALERT_TRIGGERED_OR_RESOLVED]: [ENDPOINT_GROUP] - [ENDPOINT_NAME] - [ALERT_DESCRIPTION] - [RESULT_ERRORS]"
      }
endpoints:
  - name: website
    url: "https://twin.sh/health"
    interval: 30s
    conditions:
      - "[STATUS] == 200"
      - "[BODY].status == UP"
      - "[RESPONSE_TIME] < 300"
    alerts:
      - type: custom
        failure-threshold: 10
        success-threshold: 3
        send-on-resolved: true
        description: "health check failed"

It would be great to support this at the endpoint level to allow more flexibility in custom alerting. It could be allowed to write the body in the endpoint level overwriting the body from the alerting level if defined. e.g

alerting:
  custom:
    url: "https://hooks.slack.com/services/**********/**********/**********"
    method: "POST"
    body: |
      {
        "text": "[ALERT_TRIGGERED_OR_RESOLVED]: [ENDPOINT_GROUP] - [ENDPOINT_NAME] - [ALERT_DESCRIPTION] - [RESULT_ERRORS]"
      }
endpoints:
  - name: website
    url: "https://twin.sh/health"
    interval: 30s
    conditions:
      - "[STATUS] == 200"
      - "[BODY].status == UP"
      - "[RESPONSE_TIME] < 300"
    alerts:
      - type: custom
        failure-threshold: 10
        success-threshold: 3
        send-on-resolved: true
        description: "health check failed"
      body: |
        {
          "text": "this is a custom body"
        }

Why do you personally want this feature to be implemented?

To have more flexibility in the custom alerting

How long have you been using this project?

1 week

Additional information

It would be cool to extend this feature to placeholders

    placeholders:
      ALERT_TRIGGERED_OR_RESOLVED:
        TRIGGERED: "partial_outage"
        RESOLVED: "operational"

Extending this to be configurable at the endpoint level