Support affinity, tolerations, and nodeSelector in PodTemplate

[touch-dev-null]

What would you like to be added?

Add scheduling fields (affinity, tolerations, nodeSelector) to the PodTemplate spec, as discussed in #175.

In #175, the decision was to start with PodTemplate.Metadata (labels/annotations) and expand later:

"Noting that later, we can add a new field Spec under PodTemplate, and allow the user to override items from the Pod Spec." — @ivanvc

I'd like to request adding at minimum:

• affinity (for pod anti-affinity — spreading etcd members across nodes)
• tolerations
• nodeSelector

Why is this needed?

For production etcd clusters, pod anti-affinity is essential to ensure etcd members run on different nodes. Without it, a single node failure can take down multiple etcd members and break quorum.

Example desired configuration:

apiVersion: operator.etcd.io/v1alpha1
kind: EtcdCluster
metadata:
  name: etcd
  namespace: ingress-apisix
spec:
  size: 3
  version: "v3.5.23"
  podTemplate:
    spec:
      affinity:
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            - labelSelector:
                matchExpressions:
                  - key: app.kubernetes.io/name
                    operator: In
                    values:
                      - etcd
              topologyKey: kubernetes.io/hostname
      nodeSelector:
        node-role: infra
  storageSpec:
    storageClassName: hcloud-volumes
    volumeSizeRequest: 10Gi

resources would also be a valuable addition, but scheduling fields are the most critical for production reliability.