Add support for events

[pleshakov]

• Feature: The Ingress controller now emits events when generating or updating configuration for Ingress resources, either when an Ingress resource is added or updated or the ConfigMap with NGINX configuration is added or updated. The Ingress controller emits events for Ingress resources and the ConfigMap resource. If the configuration was not successfully applied (for example, NGINX failed to validate it), the controller emits a warning event with a description of the error.
• Change: If the ConfigMap is added/updated, the Ingress controller first updates the configurations files for all Ingress resources and only then reloads NGINX. Before, the controller reloaded NGINX after updating the configuration of every Ingress resource.
• Change: NGINX Plus upstream servers are now stored in the corresponding NGINX configuration file, instead of a separate state file. The Ingress controller updates endpoints without reloading NGINX Plus, via the API, as it was before.
• Bugfix: When endpoints are removed from a Service (for example, when scaling the number of pods to 0), the Ingress controller didn't remove endpoints from NGINX Plus.
• Bugfix: When the Ingress controller removed the last upstream server in an upstream in NGINX Plus, it reported an error, when there was no error.