SIGNL4 Alert Transport

[rons4]

Please give a short description what your pull request is for

This PR adds SIGNL4 as an alert transport to LibreNMS. You can find more information here:
https://docs.signl4.com/integrations/librenms/librenms.html

DO NOT DELETE THE UNDERLYING TEXT

Please note

Please read this information carefully. You can run ./lnms dev:check to check your code before submitting.

• [x ] Have you followed our code guidelines?
• [x ] If my Pull Request does some changes/fixes/enhancements in the WebUI, I have inserted a screenshot of it.
• [x ] If my Pull Request makes discovery/polling/yaml changes, I have added/updated test data.

Screenshots can be found here: https://docs.signl4.com/integrations/librenms/librenms.html

Testers

If you would like to test this pull request then please run: ./scripts/github-apply <pr_id>, i.e ./scripts/github-apply 5926
After you are done testing, you can remove the changes with ./scripts/github-remove. If there are schema changes, you can ask on discord how to revert.

[CLAassistant]

All committers have signed the CLA.

[electrocret]

LGTM