app cr downwardAPI should support parsing spec fields

**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](https://carvel.dev/kapp-controller/docs/v0.47.x/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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