Add optional highAvailability feature for registry cache

[dergeberl]

How to categorize this PR?

/area high-availability
/kind enhancement

What this PR does / why we need it:

I reopend this PR due to some special A in the branch name of the other PR (#247)
This PR adds the optional highAvailability setting for the proxy cache. As mentioned in #246 in some cases the registry cache becomes more critical (like the proxy use case). In such cases it makes sense to scale the registry cache to more replicas. This PR uses the High Availability of Deployed Components feature to also set a working topologySpreadConstraints.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:

Release note:

Add optional setting for a registry cache to scale the `StatefulSet` to 2 replicas with the [High Availability of Deployed Components](https://gardener.cloud/docs/gardener/high-availability/#system-components) feature.

[gardener-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign ialidzhikov for approval. For more information see the Kubernetes Code Review Process.

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

Needs approval from an approver in each of these files:

• OWNERS

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

[gardener-prow]

@dergeberl: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gardener-extension-registry-cache-e2e-kind	3fe41e1	link	true	/test pull-gardener-extension-registry-cache-e2e-kind

Full PR test history. Your PR dashboard. Command help for this repository.
Please help us cut down on flakes by linking this test failure to an open flake report or filing a new flake report if you can't find an existing one. Also see our testing guideline for how to avoid and hunt flakes.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[gardener-prow]

PR needs rebase.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.