Add KEDA scaledObject option

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What does this MR do?

This MR adds the possibility of using a KEDA scaledObject for runners autoscaling.

The new template is based on https://keda.sh/docs/2.17/reference/scaledobject-spec/

Why was this MR needed?

KEDA scaledObject is more powerful than HPA : https://keda.sh/docs/2.17/scalers/

For example we can autoscale with kubernetes-workload scaler :

• our runner gitlab-runner-x has 100 concurrent jobs configured
• we deploy it with 1 minReplica
• when we have 90 executor pods with label 'managed-by-runner=gitlab-runner-x', we want to scale to authorize more jobs by adding one runner replica
• if we have 5 maxReplicas configured, it will scale again by adding one runner replica when we'll have 180, then 270, and then 360 executor pods

What's the best way to test this MR?

gitlab-runner(keda-scaled-object)$ helm unittest .

### Chart [ gitlab-runner ] .

 PASS  test deployment  tests/deployment_test.yaml
 PASS  test ingress session     tests/ingress-session_test.yaml
 PASS  test role and role-binding       tests/rbac_test.yaml
 PASS  test scaled-object       tests/scaled-object_test.yaml
 PASS  test service-account     tests/service-account_test.yaml

Charts:      1 passed, 1 total
Test Suites: 5 passed, 5 total
Tests:       39 passed, 39 total
Snapshot:    2 passed, 2 total
Time:        9.858005817s

What are the relevant issue numbers?

#562