Alertmanager support for multiple addresses

Hi,
We’re currently deploying LibreNMS with Alertmanager transport. Our Alertmanager deployment is in HA and thus there are multiple AM hosts. As far as I can see LibreNMS supports only one address for Alertmanager, it would be useful to be able to support a list of Alertmanager hosts for HA deployments.

thanks!

Related PR: https://github.com/librenms/librenms/pull/12346