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

github/fake/options.go

@@ -1,8 +1,11 @@
 package fake
 
+import "net/http"
+
 type FixedResponses struct {
 	ListRepositoryWorkflowRuns *Handler
 	ListWorkflowJobs           *MapHandler
+	ListRunners                http.Handler
 }
 
 type Option func(*ServerConfig)
@@ -25,6 +28,15 @@ func WithListWorkflowJobsResponse(status int, bodies map[int]string) Option {
 	}
 }
 
+func WithListRunnersResponse(status int, body string) Option {
+	return func(c *ServerConfig) {
+		c.FixedResponses.ListRunners = &ListRunnersHandler{
+			Status: status,
+			Body:   body,
+		}
+	}
+}
+
 func WithFixedResponses(responses *FixedResponses) Option {
 	return func(c *ServerConfig) {
 		c.FixedResponses = responses