feat: add per-namespace webhook support using namespaceSelector and WEBHOOK_PREFIX

## Enhancement description
Add per-namespace webhook support in the `cloudnative-pg` Helm chart using `namespaceSelector` and `WEBHOOK_PREFIX`.

When `config.clusterWide=false`, the chart should:
- set `WEBHOOK_PREFIX` to `-` in the operator Deployment
- create prefixed mutating/validating webhook configuration names
- scope webhook entries to the release namespace via `namespaceSelector`

When `config.clusterWide=true`, existing cluster-wide behavior must remain unchanged.

## Related work
This is the Helm chart companion for operator support tracked in cloudnative-pg/cloudnative-pg#10792.

## Scope
- update chart templates for Deployment and webhook configurations
- add chart template tests validating namespace-scoped and cluster-wide rendering