feat: Add support for using existing PriorityClass when level=0

[dibaro]

Description

This PR adds support for using existing PriorityClass resources in the cluster when level=0 and a name is provided. Previously, users could only create new PriorityClass resources or not use any priority at all.

Changes

• Modified imgproxy.resources.priorityClassName helper to return the name when level=0 and a name is provided
• Updated imgproxy.resources.explicitPriorityClassName helper to only create PriorityClass resources when level > 0
• Enhanced documentation in values.yaml to clearly explain the three usage scenarios

Usage Scenarios

1. level > 0: Creates a new PriorityClass with the specified name and assigns it to pods
2. level = 0 + name provided: Uses an existing PriorityClass with the specified name (no new PriorityClass created)
3. level = 0 + no name: No PriorityClass is used

Example

resources:
  deployment:
    priority:
      name: "my-existing-priority-class"
      level: 0

Testing

• Verified that existing PriorityClass is used when level=0 and name is provided
• Confirmed that new PriorityClass is still created when level > 0
• Ensured no PriorityClass is used when both level=0 and no name is provided
• All existing functionality remains backward compatible

Backward Compatibility

This change is fully backward compatible. All existing configurations will continue to work as before.

[gzigzigzeo]

Thank you. LGTM, will merge shortly.