What type of PR is this?

/kind feature

What this PR does / why we need it:

As a PCS user, I want to define custom environment variables (e.g. ENGINE_ID for Dynamo) that are derived from GROVE_PCLQ_POD_INDEX. This is not possible today because the operator injects GROVE_PCLQ_POD_INDEX as a direct value at pod creation time -- there is no label or field users can reference via the Kubernetes Downward API in their own env var definitions.

This PR:

• Adds a new pod label grove.io/podclique-pod-index carrying the pod's index within its PodClique.
• Changes GROVE_PCLQ_POD_INDEX from a direct-value env var to one sourced via fieldRef from that label.

This follows the same pattern already used for GROVE_PCSG_INDEX / GROVE_PCSG_NAME.

Users can now define their own env vars referencing the pod index:

env:
  - name: ENGINE_ID
    valueFrom:
      fieldRef:
        fieldPath: metadata.labels['grove.io/podclique-pod-index']

Which issue(s) this PR fixes:

Fixes #506

Special notes for your reviewer:

This is a backward-compatible change. GROVE_PCLQ_POD_INDEX still resolves to the same value at runtime -- the only difference is that it is now sourced from a pod label via fieldRef instead of being set as a direct value.

Does this PR introduce a API change?

Add `grove.io/podclique-pod-index` label to pods, allowing users to reference the pod index via the Kubernetes Downward API in custom environment variables. `GROVE_PCLQ_POD_INDEX` is now sourced from this label via fieldRef.

Additional documentation e.g., enhancement proposals, usage docs, etc.:

NONE