[alertmanager] Add Alertmanager Plugin

[ricoberger]

Is your feature request related to a problem? Please describe

It would be nice to have an overview of all open alerts from an Alertmanager instance. The plugin should support a similar feature set like the Opsgenie plugin.

Describe the solution you'd like

It should be possible to view all open and silenced alerts from a configured Alertmanager instance. A user should also be able to filter the list of shown alerts and to silence an alert via kobs.

For the silence alert function, the plugin should use a similar logic like the Opsgenie plugin, this means:

• When a user silences an alert, the name of the user should be added.
• The plugin should have a permissions section, were we can define if a user is allowed to silence alerts.