feat: Expose pvc retention policy via helm

[abhimanyu003]

What this PR does / why we need it:

Expose PVC retention policy via helm. Retain is the default value.
Info on retention policy can be found here: https://kubernetes.io/docs/concepts/workloads/controllers/statefulset/#persistentvolumeclaim-retention

Example YAML

apiVersion: mlops.seldon.io/v1alpha1
kind: Server
  name: triton
  namespace: seldon
spec:
  replicas: 1
  serverConfig: triton
  statefulSetPersistentVolumeClaimRetentionPolicy:
    whenDeleted: Retain
    whenScaled: Retain

Fixes #INFRA-1235

[sakoush]

LGTM. Left a minor comment.

Has this been tested?