Add SIGNL4 Receiver for mobile alerting

Proposal

Summary

We would like to propose adding a native SIGNL4 receiver to Alertmanager, similar to existing integrations such as PagerDuty, OpsGenie, and VictorOps.

SIGNL4 provides mobile alerting via app push notifications, SMS, and voice calls, including on-call scheduling, escalation, and collaboration capabilities.

While it is already possible to integrate SIGNL4 with Prometheus / Alertmanager using webhooks:
https://docs.signl4.com/integrations/prometheus/prometheus.html
the current approach has functional limitations compared to a native receiver implementation.

Motication

A native SIGNL4 receiver would provide improved functionality, flexibility, and a better user experience, including:

• Improved alert formatting
  • More advanced control over subject, message body, and parameters
  • Structured field mapping
• Category / Service support
  • Control over alert color, icon, and behavior
  • Better alignment with service-based routing
• Location support
  • Native support for longitude/latitude
  • Map rendering within mobile alerts
• Full alert lifecycle support
  • Easily trigger alerts
  • Properly close/resolve alerts
• Bi-directional context
  • Ability to attach additional information to enable meaningful acknowledgements from SIGNL4 back to Alertmanager

Existing Work

There has already been a previous pull request implementing this functionality, meaning that a code base for such a receiver already exists and could potentially be reused or updated.

Expected Outcome

Adding a native SIGNL4 receiver would:

• Provide feature parity with other major alerting platforms
• Improve formatting and lifecycle handling
• Reduce custom webhook configuration effort
• Deliver a better operational experience for joint Prometheus / Alertmanager / SIGNL4 users

We would be happy to support testing, feedback, or further refinement if needed.

Start by reviewing the existing PagerDuty, OpsGenie, and VictorOps receiver integrations and the previous implementation mentioned in the issue. Use the listed formatting, category, location, lifecycle, and acknowledgement requirements to define the native SIGNL4 receiver; done means those capabilities are covered without relying on the current webhook approach.