alertmanager: support for reusable templates

https://prometheus.io/docs/alerting/latest/notification_examples/#defining-reusable-templates

Please correct me if I'm wrong, but I don't see any support for defining and loading these in just yet. Reusable templates are a useful feature when configuring receivers.

It seems they could be defined in a configmap or secret in gmp-public and then the operator could take care of copying to gmp-system and volume mounting into alertmanager? I'm guessing this could be similar to how this is handled by `prometheus-operator` https://github.com/prometheus-operator/prometheus-operator/blob/main/Documentation/user-guides/alerting.md#using-a-kubernetes-secret