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

controllers/horizontal_runner_autoscaler_webhook_on_push.go

@@ -0,0 +1,24 @@
+package controllers
+
+import (
+	"github.com/google/go-github/v33/github"
+	"github.com/summerwind/actions-runner-controller/api/v1alpha1"
+)
+
+func (autoscaler *HorizontalRunnerAutoscalerGitHubWebhook) MatchPushEvent(event *github.PushEvent) func(scaleUpTrigger v1alpha1.ScaleUpTrigger) bool {
+	return func(scaleUpTrigger v1alpha1.ScaleUpTrigger) bool {
+		g := scaleUpTrigger.GitHubEvent
+
+		if g == nil {
+			return false
+		}
+
+		push := g.Push
+
+		if push == nil {
+			return false
+		}
+
+		return true
+	}
+}