refactor: migrate Template field from value to pointer type

[LikiosSedo]

Motivation

This PR addresses the template field design issue discussed in KEP-8 (#70).

Problem

KEP-8 introduces templateRef and template as logically mutually exclusive options. However, with the current value-type design:

• Zero value PodTemplateSpec{} serializes to invalid JSON that violates CRD schema
• Users would need placeholder templates even when using templateRef (bad UX)
• No clear way to express "template not set" vs "empty template"

Solution

Migrate Template from value type to pointer type (Solution 2 from the discussion).

Why pointer over placeholder approach:

• Cleaner semantics: nil = not set, avoiding placeholder code smell
• Aligns with Kubernetes conventions (large optional structs use pointers)
• Enables strict mutual exclusion validation
• Better foundation for future horizontal expansion (as noted by maintainers)

Timing:

• User base is still small, best time for breaking changes
• This PR need merge before KEP-8 implementation (as suggested by maintainers)
• Splitting API change from KEP-8 feature for easier review

Changes

API Layer

• RoleSpec.Template: corev1.PodTemplateSpec → *corev1.PodTemplateSpec
• Mark as +optional (was +kubebuilder:validation:Required)
• CRD schema updated: template no longer in required fields

Controller Logic

• Handle nil Template gracefully with zero-value fallback
• Maintains backward compatibility: existing behavior unchanged
• Internal functions updated to use pointer throughout

Test Updates

• All test code updated to use pointer initialization (&template or ptr.To())
• Approximately 10 files modified across unit tests and E2E tests

Auto-generated

• DeepCopy code regenerated
• CRD manifests updated

Breaking Change

For Go SDK Users

Before:

role.Template = corev1.PodTemplateSpec{
    Spec: corev1.PodSpec{
        Containers: []corev1.Container{...},
    },
}

After:
role.Template = &corev1.PodTemplateSpec{
    Spec: corev1.PodSpec{
        Containers: []corev1.Container{...},
    },
}

For YAML Users

No change required. YAML manifests remain compatible:

roles:
- name: role1
  template:  # Still works, now optional
    spec:
      containers:
      - name: app
        image: nginx

For KEP-8 templateRef mode (future PR):

 roles:
 - name: role1
   templateRef:
     name: base-template
   # template field can be omitted entirely

Testing

• Unit tests and E2E tests verified
• No regression in test coverage or behavior

Related

Relates to #70

This is the prerequisite API change. KEP-8 feature implementation (templateRef/templatePatch) will follow in a separate PR after community review.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[RongGu]

/gemini review

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[Syspretor]

/lgtm
/approve