text: Title: [Alerting] Concentrate public Alerting Framework components in the alerting plugin
  Body: There is some overlap between *alerting* and *triggers_actions_ui* and we should concentrate the client side APIs of the framework itself under the alerting plugin.
  
  For example, there are two alert_api.ts across both plugins, and they can be merged (they aren't exactly the same, but could reside together).