feat(alerting): custom - replace placeholders in header contents

[stendler]

Summary

This PR allows customizing headers with placeholder contents, e.g., based on ALERT_TRIGGERED_OR_RESOLVED.
Also, implemented ReplacePlaceholder to avoid repeating replace in body, url, and in a loop for the headers.

Example use case customizing ntfy:

alerting:
  custom:
    url: "https://ntfy.sh/mytopic"
    method: POST
    body: |
      Alert for [ENDPOINT_URL] with description:
      [ALERT_DESCRIPTION]
    headers:
      Firebase: "no"
      Title: "[ENDPOINT_GROUP] - [ENDPOINT_NAME]"
      Tags: "[ALERT_TRIGGERED_OR_RESOLVED]"
      Priority: 4
    placeholders:
      ALERT_TRIGGERED_OR_RESOLVED:
        TRIGGERED: "rotating_light"
        RESOLVED: "white_check_mark"
    default-alert:
      failure-threshold: 2
      success-threshold: 3
      send-on-resolved: true

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.