app cr downwardAPI should support parsing spec fields

[jorgemoralespou]

What steps did you take:
Create an App CR and wanted to reference a image subPath of one of the fetched artifacts with:

template:
  - ytt:
       valuesFrom:
          - downwardAPI:
              items:
                name: subpath
                fieldPath: spec.fetch[?(@.image)].image.subPath

What happened:
I got:

Preparing template values: spec is not found

What did you expect:
I expected to get the subpath from proper resolution of downwarAPI.

Anything else you would like to add:
I've seen in the detailed App spec this comment:

# fieldPath accepts relaxed jsonpath to select metadata fields from the AppCR (name, namespace, uid, labels, annotations)

Related thread in k8s slack: https://kubernetes.slack.com/archives/CH8KCCKA5/p1696521588882029

Environment:

• kapp Controller version (execute kubectl get deployment -n kapp-controller kapp-controller -o yaml and the annotation is kbld.k14s.io/images): 0.48.0
• Kubernetes version (use kubectl version) 1.27

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[github-actions]

This issue is being marked as stale due to a long period of inactivity and will be closed in 5 days if there is no response.

[jorgemoralespou]

/remove-stale

[github-actions]

This issue is being marked as stale due to a long period of inactivity and will be closed in 5 days if there is no response.