Add optional hostUsers field to the KEDA pods

[somaz94]

Adds an opt-in hostUsers field to the operator, metrics server and admission
webhooks pods so KEDA's components can run in their own user namespace
(graduated in Kubernetes 1.36). Mirrors the existing useHostNetwork toggle.

Unset by default, so existing renders are byte-identical; setting
operator.hostUsers / metricsServer.hostUsers / webhooks.hostUsers to
true or false emits the field on the matching pod only.

Validation (local)

• helm lint keda: 0 failed
• helm template: default omits the field (no diff for current users); set to
  false/true renders it on the correct pod(s) only
• helm install on a kind v1.34 cluster with hostUsers=true: operator, metrics
  server and webhooks pods all Running, field present on the live pod specs
• apiserver accepts hostUsers=false (deployment + new ReplicaSet created);
  running a pod with false additionally needs the node to support user namespaces

Checklist

• I have verified that my change is according to the deprecations & breaking changes policy
• Commits are signed with Developer Certificate of Origin (DCO)
• README is updated with new configuration values
• A PR is opened to update KEDA core — not applicable (chart-only opt-in value, default unset = no change to the static core manifests)

Fixes #879