feat: allow setting init containers for cluster-autoscaler

[hegerdes]

What type of PR is this?

/kind feature

What this PR does / why we need it:

It adds the ability to run init (and sidecar) containers within the cluster-autoscaler deployment when installing via helm. A lot of providers depend on configuration options and files depending on their cloud provider. A init container or sidecar container would allow to execute any logic in order to get or create the autoscaler provider configuration dynamically.

Which issue(s) this PR fixes:

Fixes #
Not existing yet

Special notes for your reviewer:

Example and test usage:

helm template -f myvalue.yaml -s templates/deployment.yaml

# myvalue.yaml 
initContainers:
  - name: get-cloud-conf
    image: alpine:latest
    # Optionally make it a sidecar
    # restartPolicy: Always
    command: ["sh", "-c", "curl -sl https://my-cloud-conf.com/user-data -o /opt/user-data/scale.conf"]
    volumeMounts:
      - name: user-data
        mountPath: /opt/user-data

extraVolumeMounts:
  - name: user-data
    mountPath: /opt/user-data

extraVolumes:
  - name: user-data
    emptyDir: {}

autoDiscovery:
  clusterName: demo

Does this PR introduce a user-facing change?

feat: allow setting init containers via helm for cluster-autoscaler

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: hegerdes
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[k8s-ci-robot]

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