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:

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

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[gardener-prow]

PR needs rebase.

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[ialidzhikov]

@dergeberl, thanks for the PR and sorry for the delay! We will try to provide a review next week!

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/lifecycle stale

[gardener-ci-robot]

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[timebertt]

Sorry for the delay, @dergeberl will soon return from vacation and continue working on this PR.
/remove-lifecycle rotten

[gardener-ci-robot]

The Gardener project currently lacks enough active contributors to adequately respond to all PRs.
This bot triages PRs according to the following rules:

• After 15d of inactivity, lifecycle/stale is applied
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/lifecycle stale

[dimitar-kostadinov]

/remove-lifecycle stale

[gardener-prow]

@dergeberl: The following tests 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-unit	996886f	link	true	/test pull-gardener-extension-registry-cache-unit
pull-gardener-extension-registry-cache-check-renovate-config	996886f	link	true	/test pull-gardener-extension-registry-cache-check-renovate-config

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-ci-robot]

The Gardener project currently lacks enough active contributors to adequately respond to all PRs.
This bot triages PRs according to the following rules:

• After 15d of inactivity, lifecycle/stale is applied
• After 15d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 7d of inactivity since lifecycle/rotten was applied, the PR is closed

You can:

• Mark this PR as fresh with /remove-lifecycle stale
• Mark this PR as rotten with /lifecycle rotten
• Close this PR with /close

/lifecycle stale

[dimitar-kostadinov]

/remove-lifecycle stale