feat(aws-eks): managed NodeGroup disk encryption

Encrypt EBS volumes backing Managed NodeGroup EC2 instances.

### Use Case

We want to enforce security best practices when using EKS Managed NodeGroups.

### Proposed Solution

Add a boolean parameter "diskEncrypted" in [managed-nodegroup](https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-eks/lib/managed-nodegroup.ts#L242).

Requires EKS NodeGroup CFN to add a boolean encryption parameter as well.

### Other

- [AWS EKS NodeGroup CFN specification](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-eks-nodegroup.html)

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request