feat: Repository-wide RunnerDeployment Autoscaling (#57)

* feat: Repository-wide RunnerDeployment Autoscaling

This adds `maxReplicas` and `minReplicas` to the RunnerDeploymentSpec. If and only if both fields are set, the controller computes and sets desired `replicas` automatically depending on the demand.

The number of demanded runner replicas is computed by `queued workflow runs + in_progress workflow runs` for the repository. The support for organizational runners is not included.

Ref https://github.com/summerwind/actions-runner-controller/issues/10

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

@@ -46,8 +46,21 @@ spec:
         spec:
           description: RunnerReplicaSetSpec defines the desired state of RunnerDeployment
           properties:
+            maxReplicas:
+              description: MinReplicas is the maximum number of replicas the deployment
+                is allowed to scale
+              type: integer
+            minReplicas:
+              description: MinReplicas is the minimum number of replicas the deployment
+                is allowed to scale
+              type: integer
             replicas:
               type: integer
+            scaleDownDelaySecondsAfterScaleOut:
+              description: ScaleDownDelaySecondsAfterScaleUp is the approximate delay
+                for a scale down followed by a scale up Used to prevent flapping (down->up->down->...
+                loop)
+              type: integer
             template:
               properties:
                 metadata:
@@ -6717,13 +6730,20 @@ spec:
                   type: object
               type: object
           required:
-          - replicas
           - template
           type: object
         status:
           properties:
             availableReplicas:
               type: integer
+            desiredReplicas:
+              description: Replicas is the total number of desired, non-terminated
+                and latest pods to be set for the primary RunnerSet This doesn't include
+                outdated pods while upgrading the deployment and replacing the runnerset.
+              type: integer
+            lastSuccessfulScaleOutTime:
+              format: date-time
+              type: string
             readyReplicas:
               type: integer
           required: