Introduce collections support

[a-hilaly]

This PR introduces collections support in RGDs through a new forEach field, enabling dynamic creation of multiple resources based on iterative expressions.

• Added forEach field to RGDs spec allowing iteration over both arrays and maps
• Implemented CEL based collection evaluation with support for filtering and mapping
• Added collection aware resource reconciliation in the controller
• Introduced each context variables (item, index, length, key, value) for template access

Example usage:

resources:
  - id: replicaSet
    forEach: ${schema.spec.roles.filter(x, x.substring(0, 4) != 'test')}
    template:
      apiVersion: v1
      kind: Pod
      metadata:
        name: ${schema.spec.name + '-' + each.item}
      spec:
        containers:
          - name: ${schema.spec.name + '-' + each.item}
            image: ${schema.spec.image}

[barney-s]

Do we want to start with a design doc first ?

[k8s-triage-robot]

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[brandonjbjelland]

This is at the top of my wish list for kro - thank you for this work and anxiously awaiting its arrival!

[a-hilaly]

@brandonjbjelland we're excited as well! i did open a proposal describing the collection semantics and possible use cases, your feedback would be highly valuable! => #679

[vast-raymond-page]

Instead of a "simple" case, can we have forEach be an array of collections as the only supported mechanic? I really like the idea of an array of collections to allow multiple iterators running over every combination of collections like discussed here:
#679 (comment)

  forEach:
    - collection: ${spec.regions}
      name: "region"
    - collection: ${spec.zones}
      name: "zone"
  template:
    kind: Pod
    metadata:
      name: ${'pod-' +region.entry+ '-' +  zone.entry}

[k8s-ci-robot]

PR needs rebase.

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