[helm] Add pod scheduling primitives (affinity, nodeSelector, tolerations, topologySpreadConstraints)

[fresh-borzoni]

Search before asking

• I searched in the issues and found nothing similar.

Description

Production Kubernetes deployments often need to spread tablet server replicas across availability zones and nodes to survive zone/node failures, pin workloads to specific node pools, or allow scheduling on tainted nodes.

Without these primitives exposed in the chart, users have to fork the templates or patch the rendered manifests.

Willingness to contribute

• I'm willing to submit a PR!