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

github/fake/options.go

@@ -0,0 +1,28 @@
+package fake
+
+type FixedResponses struct {
+	listRepositoryWorkflowRuns FixedResponse
+}
+
+type FixedResponse struct {
+	Status int
+	Body   string
+}
+
+func (r FixedResponse) handler() handler {
+	return handler{
+		Status: r.Status,
+		Body:   r.Body,
+	}
+}
+
+type Option func(responses *FixedResponses)
+
+func WithListRepositoryWorkflowRunsResponse(status int, body string) Option {
+	return func(r *FixedResponses) {
+		r.listRepositoryWorkflowRuns = FixedResponse{
+			Status: status,
+			Body:   body,
+		}
+	}
+}