separate statefulset templating for arbiters and members

[sina-grz]

Summary:

I previously described the problems and possible solutions in this issue:
#1737

This branch implements the second proposed solution: Context-Aware Default Templates.
The goal is to avoid unnecessary resource waste by providing separate defaults for arbiters vs. data members while keeping customization consistent.

Implementation:

• Pass an isArbiter boolean to the build function
• Apply arbiter-optimized defaults when isArbiter=true:
  • No PVC
  • Minimal resources
• Maintain current behavior for data members
• Keep spec.statefulSet for member customization only

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