Validate cluster stacks

[janiskemper]

/kind feature

Describe the solution you'd like
Add validation of cluster stacks as feature to csctl. Some validation might be before the generation, some might be after the generation of the assets.

Validate the following

• Helm template
• parsing clusteraddon.yaml
• CEL expressions in clusteraddon.yaml

Anything else you would like to add:
There is also some advanced validating we could do. That requires some further setup with a running cluster for example.

• Applying objects from helm charts (only possible with running clusters)
• Validating CEL expressions even further (requires CRDs of objects), e.g. by checking certain keys exist or not