Add fieldRef

[stevendborrelli]

Description of your changes

Adds a fieldRef to shellEnvVars. A fieldRef is like a valueRef, but it allows the ability to return a null string or a defaultValue if the field is not present.

Fixes #75

An image is available for testing at index.docker.io/steve/function-shell:fieldref.

shellEnvVars:
- key: EXAMPLE_VAR
  type: FieldRef
  fieldRef:
    policy: "Optional"
    path: "status.field"
    defaultValue: "NotSet"

Testing

Ran the example/functionRef composition in a cluster. It correctly sets default fields and reads from the XR status:

apiVersion: example.crossplane.io/v1
kind: XR
metadata:
  annotations:
    kubectl.kubernetes.io/last-applied-configuration: |
      {"apiVersion":"example.crossplane.io/v1","kind":"XR","metadata":{"annotations":{},"name":"example-echo"},"spec":{"field":"hello from the XR"}}
  creationTimestamp: "2025-08-13T16:16:54Z"
  finalizers:
  - composite.apiextensions.crossplane.io
  generation: 5
  labels:
    crossplane.io/composite: example-echo
  name: example-echo
  resourceVersion: "62783"
  uid: c5439354-df24-4494-af80-65375852ea03
spec:
  compositionRef:
    name: shell-example
  compositionRevisionRef:
    name: shell-example-c375c75
  compositionUpdatePolicy: Automatic
  field: hello from the XR
  resourceRefs: []
status:
  atFunction:
    shell:
      stderr: ""
      stdout: |-
        echo: hello from the XR optional: defaultValue from Composition
        status.atFunction.shell.stdout is populated
  conditions:
  - lastTransitionTime: "2025-08-13T16:18:14Z"
    observedGeneration: 5
    reason: ReconcileSuccess
    status: "True"
    type: Synced
  - lastTransitionTime: "2025-08-13T16:18:14Z"
    observedGeneration: 5
    reason: Available
    status: "True"
    type: Ready

I have:

• Read and followed Crossplane's
  contribution process. Also see docs.
• Added or updated unit tests for my change.