Werk #13139: Add Alertmanager special agent and check plugins

Add special agent and check plugins for Alertmanager.

The special agent retrieves information from the Alertmanager rules API. It retrieves information about:

• alert rule groups
• alert rules
  1. name
  2. status
  3. severity
  4. message

The check plugin monitors

• Individual alert rules
• Alert rule groups
• Alert rule summary

For more information look at the respective manpages

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