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.:

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[k8s-ci-robot]

Hi @hegerdes. Thanks for your PR.

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[jackfrancis]

/lgtm

/hold

let's merge #6780 first

@hegerdes FYI there's really no way to avoid doing these merges serially, you'll have to rebase and then revise the chart version

[gjtempleton]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: gjtempleton, hegerdes, jackfrancis

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• charts/OWNERS [gjtempleton]

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[jackfrancis]

/hold cancel