feat: HorizontalRunnerAutoscaler Webhook server (#282)

* feat: HorizontalRunnerAutoscaler Webhook server

This introduces a Webhook server that responds GitHub `check_run`, `pull_request`, and `push` events by scaling up matched HorizontalRunnerAutoscaler by 1 replica. This allows you to immediately add "resource slack" for future GitHub Actions job runs, without waiting next sync period to add insufficient runners.

This feature is highly inspired by https://github.com/philips-labs/terraform-aws-github-runner. terraform-aws-github-runner can manage one set of runners per deployment, where actions-runner-controller with this feature can manage as many sets of runners as you declare with HorizontalRunnerAutoscaler and RunnerDeployment pairs.

On each GitHub event received, the webhook server queries repository-wide and organizational runners from the cluster and searches for the single target to scale up. The webhook server tries to match HorizontalRunnerAutoscaler.Spec.ScaleUpTriggers[].GitHubEvent.[CheckRun|Push|PullRequest] against the event and if it finds only one HRA, it is the scale target. If none or two or more targets are found for repository-wide runners, it does the same on organizational runners.

Changes:

* Fix integration test
* Update manifests
* chart: Add support for github webhook server
* dockerfile: Include github-webhook-server binary
* Do not import unversioned go-github
* Update README

charts/actions-runner-controller/values.yaml

@@ -107,4 +107,47 @@ priorityClassName: ""
 env: {}
   # http_proxy: "proxy.com:8080"
   # https_proxy: "proxy.com:8080"
-  # no_proxy: ""
+  # no_proxy: ""
+
+githubWebhookServer:
+  enabled: false
+  labels: {}
+  replicaCount: 1
+  syncPeriod: 10m
+  secret:
+    enabled: false
+    ### GitHub Webhook Configuration
+    #github_webhook_secret_token: ""
+  image:
+    repository: summerwind/actions-runner-controller
+    # Overrides the manager image tag whose default is the chart appVersion if the tag key is commented out
+    tag: "latest"
+    pullPolicy: IfNotPresent
+  imagePullSecrets: []
+  nameOverride: ""
+  fullnameOverride: ""
+  serviceAccount:
+    # Specifies whether a service account should be created
+    create: true
+    # Annotations to add to the service account
+    annotations: {}
+    # The name of the service account to use.
+    # If not set and create is true, a name is generated using the fullname template
+    name: ""
+  podAnnotations: {}
+  podSecurityContext: {}
+  # fsGroup: 2000
+  securityContext: {}
+  resources: {}
+  nodeSelector: {}
+  tolerations: []
+  affinity: {}
+  priorityClassName: ""
+  service:
+    type: NodePort
+    ports:
+    - port: 80
+      targetPort: 8000
+      protocol: TCP
+      name: http
+      #nodePort: someFixedPortForUseWithTerraformCdkCfnEtc