separate arbiter and members resource creation

[sina-grz]

Summary

I previously described the problems and possible solutions in this issue:
#416
mongodb/mongodb-kubernetes-operator#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.

Proof of Work

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

Checklist

• Have you linked a jira ticket and/or is the ticket in the title?
• Have you checked whether your jira ticket required DOCSP changes?
• [ ✅] Have you added changelog file?
  • use skip-changelog label if not needed
  • refer to Changelog files and Release Notes section in CONTRIBUTING.md for more details

[vinilage]

Thank you very much @sina-grz for the contribution. We will review the PR and get back to you.

[codeowners-service-app]

Assigned Julien-Ben for team kubernetes-hosted because fealebenpae is out of office.
Assigned viveksinghggits for team kubernetes-hosted because lucian-tosa is out of office.

[PeterC89]

IMO it would be nice if it were possible to define a separate spec for arbiters.
For example, we have a need to run an arbiter on a specific node, which can't be done under the current setup, or via the option proposed in this PR.
Sadly it's not a feature I can contribute to as I'm not a Go developer 😢