feat(sources): extract Namespace annotations in kubernetes_logs

[srstrickland]

Summary

kubernetes_logs currently enriches events with the owning Namespace's labels (kubernetes.namespace_labels) but has no equivalent for Namespace annotations. This adds namespace_annotation_fields.namespace_annotations, mirroring the existing namespace_labels handling in namespace_metadata_annotator.rs and the pod_annotations field already supported on Pods. This lets users store larger/more complex values in a namespace annotation (e.g. multi-value routing config, a team/oncall identifier) without hitting the 63-character Kubernetes label-value limit.

Closes #21428.

Vector configuration

sources:
  kubernetes_logs:
    type: kubernetes_logs
    namespace_annotation_fields:
      namespace_annotations: .kubernetes.namespace_annotations
      # or "" to suppress, or a custom path, same as namespace_labels

Given a Namespace annotated with:

metadata:
  annotations:
    mycompany.com/oncall: platform-team

events from pods in that namespace now carry:

{
  "kubernetes": {
    "namespace_annotations": {
      "mycompany.com/oncall": "platform-team"
    }
  }
}

How did you test this PR?

• Added unit test coverage in namespace_metadata_annotator.rs.
• Built a one-off image & manually tested in a live k8s cluster; works as expected.

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

(Added: changelog.d/k8s_namespace_annotations.feature.md.)

References

• Closes: Kubernetes Source: Add namespace annotations to event metadata #21428

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