Add HRA printer column "SCHEDULE" (#561)

Adds a column to help the operator see if they configured HRA.Spec.ScheduledOverrides correctly, in a form of "next override schedule recognized by the controller":

```
$ k get horizontalrunnerautoscaler
NAME                            MIN   MAX   DESIRED   SCHEDULE
actions-runner-aos-autoscaler   0     5     0
org                             0     5     0         min=0 time=2021-05-21 15:00:00 +0000 UTC
```

Ref https://github.com/actions-runner-controller/actions-runner-controller/issues/484

config/crd/bases/actions.summerwind.dev_horizontalrunnerautoscalers.yaml

@@ -18,6 +18,9 @@ spec:
   - JSONPath: .status.desiredReplicas
     name: Desired
     type: number
+  - JSONPath: .status.scheduledOverridesSummary
+    name: Schedule
+    type: string
   group: actions.summerwind.dev
   names:
     kind: HorizontalRunnerAutoscaler
@@ -265,6 +268,11 @@ spec:
                 which is updated on mutation by the API Server.
               format: int64
               type: integer
+            scheduledOverridesSummary:
+              description: ScheduledOverridesSummary is the summary of active and
+                upcoming scheduled overrides to be shown in e.g. a column of a `kubectl
+                get hra` output for observability.
+              type: string
           type: object
       type: object
   version: v1alpha1