Support extra pod spec for user placeholder pods

Add configuration `scheduling.userPlaceholder.extraPodSpec` for arbitrary extra pod spec for the user placeholder statefulset.

This change arose from a desire to add pod topology spread constraints to the placeholder pods. Our singleuser pods run in different availability zones at runtime, as their corresponding persistent volumes are created in different availability zones. Adding pod topology spread constraints across zones helps improving singleuser pod startup time in different availability zones.

For example,
```
scheduling:
  userPlaceholder:
    extraPodSpec:
      topologySpreadConstraints:
        - maxSkew: 1
          topologyKey: zone
          whenUnsatisfiable: DoNotSchedule
          labelSelector:
            matchLabels:
              foo: bar
```

Author: Xi Chen

jupyterhub/templates/scheduling/user-placeholder/statefulset.yaml

@@ -77,4 +77,7 @@ spec:
           securityContext:
             {{- . | toYaml | nindent 12 }}
           {{- end }}
+      {{- with .Values.scheduling.userPlaceholder.extraPodSpec }}
+      {{- . | toYaml | nindent 6 }}
+      {{- end }}
 {{- end }}

jupyterhub/values.schema.yaml

@@ -1526,6 +1526,7 @@ properties:
           2. proxy.chp.extraPodSpec
           3. proxy.traefik.extraPodSpec
           4. scheduling.userScheduler.extraPodSpec
+          5. scheduling.userPlaceholder.extraPodSpec
 
           One real-world use of these settings is to enable host networking. For
           example, to configure host networking for the hub pod, add the
@@ -2648,6 +2649,7 @@ properties:
               Unless specified here, the placeholder pods will request the same
               resources specified for the real singleuser pods.
           containerSecurityContext: *containerSecurityContext-spec
+          extraPodSpec: *extraPodSpec-spec
       corePods:
         type: object
         additionalProperties: false

jupyterhub/values.yaml

@@ -600,6 +600,7 @@ scheduling:
       seccompProfile:
         type: "RuntimeDefault"
     resources: {}
+    extraPodSpec: {}
   corePods:
     tolerations:
       - key: hub.jupyter.org/dedicated

tools/templates/lint-and-validate-values.yaml

@@ -521,6 +521,14 @@ scheduling:
     revisionHistoryLimit: 1
     replicas: 1
     resources: *resources
+    extraPodSpec:
+      topologySpreadConstraints:
+        - maxSkew: 1
+          topologyKey: zone
+          whenUnsatisfiable: DoNotSchedule
+          labelSelector:
+            matchLabels:
+              foo: bar
   corePods:
     tolerations:
       - key: mock-taint-key-corePods