Introduce common PrometheusRule to validate internal feature of Kestra

[tchiotludo]

For now, we don't provide any alert that could be useful for our customer to validate internal & functional metrics. We should add some PrometheusRule that will cover:

• Does the scheduler schedule some workload
• Does the queues are growing
• Does the executor manage some executions
• Does the worker have long running tasks
• ...

We could start from internal alert we have on the cloud