feat(ec2): ipv6AddressCount property for an instance (aws#31076)

### Issue # (if applicable)

Closes aws#31075.

### Reason for this change

AWS CDK cannot configure `ipv6AddressCount` property but cloudformation can do that.

### Description of changes

- Add `ipv6AddressCount` to `InstanceProps`
- Check whether `ipv6AddressCount` is a positive integer

### Description of how you validated changes

Add both unit and integ tests

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*