config: only allow confidential instances on stackit

[h3adex]

Context

When creating a confidential Kubernetes cluster on Stackit, customers have occasionally confused confidential flavor IDs with non-confidential ones which resulted in a "stuck" creation period.

Proposed Change(s)

I have implemented an instance type check for the Stackit cloud provider to prevent this confusion.

Related Issue

#3438

Additional Info

I have rebuilt the Constellation CLI and confirmed that the change works correctly on a Linux x86 machine.
Using bazel build //cli:cli_oss

Checklist

• Run the E2E tests that are relevant to this PR's changes

[CLAassistant]

All committers have signed the CLA.

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[msanft]

We need an if provider.OpenStack != nil here, as there's more scenarios where we'll have case variant.QEMUVTPM{}, variant.QEMUTDX{}, such as MiniConstellation, and else we'd run into a nil panic there here when dereferencing provider.OpenStack.

We might also want "success" test case for these variants without the OpenStack / STACKIT case here.

[h3adex]

I've added a provider check and two test cases. Although I'm not entirely sure which provider Miniconstellation is using, I made an educated guess: ProviderConfig{QEMU: &QEMUConfig{}} with the variant variant.QEMUTDX{}.

[msanft]

The failing CI is somewhat expected on your branch, as the required tokens aren't available there. I'll verify that these succeed on my machine once we're ready to merge, so don't worry about fixing that for now. :)

[msanft]

Thanks a lot!

[msanft]

All tests succeed, merging.