[Serverless] Adding config to disable authentication on task manager …

…background worker utilization API (#159505)

## Summary

Until [this issue](#153720) is
resolved, this config flag allows us to access the task manager
background worker utilization API in serverless to support autoscaling
of background task deployments

## To Verify

Run es: `yarn es snapshot`
Run serverless on this branch: `yarn serverless-es`

Verify you see the following warning in the logs:
```
[2023-06-12T12:47:19.641-04:00][WARN ][plugins.taskManager] Disabling authentication for background task utilization API
```

and you can access `/api/task_manager/_background_task_utilization`
without logging in

config/serverless.yml

@@ -39,3 +39,6 @@ server.securityResponseHeaders.disableEmbedding: true
 
 # Enforce single "default" space
 xpack.spaces.maxSpaces: 1
+
+# Allow unauthenticated access to task manager utilization API for auto-scaling
+xpack.task_manager.unsafe.authenticate_background_task_utilization: false