[Reporting] Ensure reports:monitor task is never "unrecognized"

[tsullivan]

Kibana version: 7.14+

When the reporting queue system was replaced with ESQueue to use Task Manager, a reports:monitor task was created as a way to add self-healing when Kibana crashes or restarts in the process of executing a report.

If this happens, the status of the report would stay in "processing" indefinitely if it weren't for the report:execute task which looks for these kinds of stuck jobs.

Problem: If the report:execute task returns an error, Task Manager reschedules it with a time buffer. These buffers can become very large. When this happens, Reporting is not able to self-heal by rescheduling stuck jobs.

Problem: the reports:monitor task is vulnerable to Kibana instances joining the cluster with the Reporting plugin disabled.

Having a mismatch of plugins enabled in different Kibana instances is not supported: it causes problems especially for plugins that are responsible for server background tasks.

When this scenario happens, the instance with Reporting disabled will not recognize the reports:monitor task. Internal logic in Task Manager updates the status of the task to unrecognized, which dictates that this task should stop running.

Workaround
Make sure that in all of the instances, the same plugins are enabled - Reporting should be enabled on every instance. If you need to ensure that only one instance is able to claim jobs, set xpack.reporting.queue.pollEnabled: false in the configuration of all instances except that one. You can disable the Reporting UI features on a per-user and per-space basis by using application feature controls. See: https://www.elastic.co/guide/en/kibana/current/secure-reporting.html#grant-user-access

Requirements:

1. The Reporting plugin should handle xpack.reporting.enabled: false with minimal custom logic to register the reports:monitor task.
2. The Diagnostic tool should warn the user if there is any problems with the state of the reports:monitor task.

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServicesUx)

[tsullivan]

Replacing with #120995