[FEATURE] Improve detection of alerting plugin in cluster

[amitgalitz]

Is your feature request related to a problem?

Currently we have an optional dependency on the alerting plugin. Users are able to click on a Set up alerts button in the anomaly detector details page in order to create a monitor on top of that detector. This button sends you to the alerting plugin's "create monitor" page. We should have a better mechanism to know if the alerting plugin is present in the cluster in order to improve the user experience since now it would send to a broken page if there is no alerting plugin. This is also relevant for the feature anywhere project where a monitor can be created through a flyout on top of detector.

What solution would you like?

We should implement a cleaner mechanism by listing alerting as an optional plugin and updating the state in setup()/start(). After implementing that we should decide if we want to hide the set up alerts if alerting is present or make a difference decision on the experience then what we currently have.