fix: validate capability configuration before cluster creation

[atharvaHJoshi]

Fixes #8701

What this PR does

Prevents a nil-pointer panic (SIGSEGV during eksctl create cluster) when an
invalid/misspelled AWS IDC configuration causes the ArgoCD AWSIDC field to remain
nil.

Root cause: config parsing performs a case-insensitive YAML unmarshal but re-decodes
case-sensitively via the Kubernetes serializer. A typo such as awsIDC (instead of
awsIdc) therefore leaves ArgoCD.AWSIDC nil, and the create-cluster path never ran
Capability.Validate(), so convertConfiguration dereferenced a nil pointer.

Changes

• Validate cluster capabilities during cluster config validation
  (NewCreateClusterLoader).
• Add a nil guard in capability conversion that returns an actionable error:
  awsIdc configuration is required for ARGOCD capability.
• Add regression tests (render with AWSIDC set; clean error when missing).
• While in configfile.go, also swap the deprecated reflect.Ptr alias for
  reflect.Pointer (fixes a golangci-lint govet finding).

Validation

• go build ./...
• golangci-lint run --timeout=30m ./pkg/...
• go test ./pkg/cfn/...
• go test ./pkg/ctl/cmdutils/...

Pre-existing, unrelated failures in pkg/info and pkg/iam/oidc are environmental
(kubectl/cfssl missing locally) and remain unchanged.

[github-actions]

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