Refactor updates to `spec` to use patches and Server-Side Apply (SSA)

[Zerpet]

Summary

The operator currently updates spec fields in the RabbitmqCluster custom resource using full Update() calls, and in some cases performs updates even when no change is needed. This can:

• Cause unnecessary updates
• Take field ownership from users or tools such as Helm (see issue reconcileOperatorDefaults performs unnecessary full Update on RabbitmqCluster CR, causing Helm SSA conflicts #2098)
• Lead to conflicts, especially with fields that have both int and string representations (e.g., resource quantities)

Proposal

• Refactor all operator logic updating spec to use targeted patches with Server-Side Apply (SSA)
• Assign granular field ownership (with appropriate fieldManager names) only to the fields the operator is responsible for
• Avoid taking ownership of fields the operator does not intend to manage
• Update only fields that truly need to change, bypassing updates when the desired and current state are identical

Benefits

• Minimizes risk of SSA and Helm/Argo conflicts
• Ensures clean and predictable field ownership
• Reduces risk of breaking user expectations by overwriting fields
• Streamlines and documents proper patch and field ownership usage for contributors

Context

This change follows the lessons from issue #2098 (SSA conflicts on Helm upgrades) and recommendations emerging from recent code and process discussions. See Kubernetes documentation on Server-Side Apply for reference.

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See also: "Move defaulting logic to a mutating webhook" (separate proposal for admission-time defaults).