Support Kubernetes Secret references for EC2NodeClass userData

Description

What problem are you trying to solve?

EC2NodeClass.spec.userData currently only accepts an inline string. This requires the complete user-data payload to be stored in the EC2NodeClass, which is commonly committed to git and deployed as a non-secret k8s resource.

We'd like to get user data from a key in a k8s secret. This would allow user-data content to be managed through existing tooling such as External Secrets without copying the resolved value into the EC2NodeClass manifest.

Something like:

  spec:
    userDataRef:
      name: bottlerocket-user-data
      key: userData

How important is this feature to you?

We can't use karpenter without something like this

Happy to contribute to implementing the feature as we're testing it internally.

Give me 1 and I'll submit a PR

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