Handle cluster-autoscaler Deployment in generic controlplane mutator

[ialidzhikov]

/area auto-scaling
/kind enhancement

What this PR does / why we need it:
When our forked cluster-autoscaler adopts a version of the upstream cluster-autoscaler that contains kubernetes/autoscaler#4539 (or any of its backports) we have to specify the CSI feature gates for the cluster-autoscaler Deployment. As the CSI feature gates are different for the different cloud providers we have to enhance the generic controlplane mutator and allow provider extensions to mutate the cluster-autoscaler Deployment in order to add the CSI feature gates.

Which issue(s) this PR fixes:
Part of #5064

Release note:

Provider extensions using the generic `controlplane` mutator webhook can now mutate the `cluster-autoscaler` Deployment by implementing the `EnsureClusterAutoscalerDeployment` function. This is required in the context of https://github.com/kubernetes/autoscaler/issues/4517 - cluster-autoscaler supports `--feature-gates` flag and provider extensions have to mutate the cluster-autoscaler Deployment to add the CSI related feature flags to it.

[rfranzke]

/lgtm
/approve

[gardener-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rfranzke

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

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [rfranzke]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment