Add CallHeim to Alertmanager webhook receiver integrations

[labeedsoft-cloud]

@RichiH

Adds CallHeim to the Alertmanager webhook receiver list. CallHeim is an on-call and incident management service that receives Alertmanager notifications through a standard webhook_configs receiver; the linked page documents the receiver configuration, send_resolved, and routing.

Example receiver:

receivers:
  - name: callheim
    webhook_configs:
      - url: '<your CallHeim integration URL>'
        send_resolved: true

I work on CallHeim.