WIP - KKP User Cluster Management with Terrafrom Rest API and Cluster CRDs

[toschneck]

closes https://github.com/kubermatic/sig-cs-infra/issues/62

What this PR does / why we need it:

Shows the ability how KKP and Terraform could interact. The Provider is considered as example to extend it to the custom needs and cloud providers. As first example kubevirt is added

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

- Adding KKP REST API Terraform provider (Scope: Example Implementation to extend to custom needs)
- Adding Examples How to manage KKP User Cluster with Cluster CRDs at the seed level

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[dharapvj]

Suggested change

	Another option is to manage the [`ClusterTempalte`](https://docs.kubermatic.com/kubermatic/v2.23/references/crds/#clustertemplate) object. Therefore, a non initialized template get created and separate instance object creates a copy of it. **BUT** any change to the clustertempalte will **NOT** get applied to the instance.
	Another option is to manage the [`ClusterTemplate`](https://docs.kubermatic.com/kubermatic/v2.23/references/crds/#clustertemplate) object. Therefore, a non initialized template get created and separate instance object creates a copy of it. **BUT** any change to the clustertemplate will **NOT** get applied to the instance.

[dharapvj]

May we should add few more things via kubectl :

1. Addon and AddonConfig CR
2. ApplicationInstallation CR
3. User-MLA Alerts (No CR I guess. Gets applied directly in Cortex?)
4. User-MLA AlertManager config (AlertManager CR)

I can try to add these after your PR is merged