Allow disabling default alert rules

[awnns]

Enhancement Proposal

In some scenarios it is not desirable to have the default alert rules from the operator be sent to the cos lite stack. The charm can pollutes alertmanager with hundreds of alerts and prevent proper administration and use of the user's own alerts. Even for modest deployments, the grafana agent operator can deposit many hundreds of alerts.

Even if this situation were to improve, it would be highly desirable for responsible users to be able to disable the default alerts in the presence of the user's own suite of alerts through some configuration option that hinders grafana agent from sending its default alert rules to alert manager.