[helm] Add Support for Generating Specific Priority Classes in Helm Chart

[pkoutsovasilis]

Describe the enhancement:
Extend the Helm chart to support the creation of specific Kubernetes PriorityClasses. This will allow users to define and manage the priority of agent pods, ensuring that critical workloads are given appropriate precedence in a resource-constrained environment.

Describe a specific use case for the enhancement or feature:
In scenarios where multiple workloads are running in the same Kubernetes cluster, it is crucial to prioritize elastic-agent pods that perform essential tasks. By configuring specific PriorityClasses through the Helm chart, users can ensure that important agent workloads receive higher scheduling priority compared to less critical processes, optimizing resource allocation and maintaining system reliability.

What is the definition of done?

• The Helm chart can define and generate custom PriorityClasses based on user configuration.
• Documentation is updated to guide users on how to set and apply these PriorityClasses.

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)