New Integration: SIGNL4 for Mobile Alerting and Incident Response

### Problem Statement

Add [SIGNL4](https://www.signl4.com/) as additional integration for mobile alerting, notification and incident response.

### Solution Brainstorm

I would be happy to help with code or with a pull request. For two-way integration (acknowledgement, annotation) it would be possible to add a connector app at SIGNL4 side.
Thanks a lot
Ron

PS: There is a simple webhook API available for triggering (and for resolving) alerts: https://connect.signl4.com/webhook/docs/index.html