fix(ec2): allow NAT instance to associate public IP (#31812)

### Issue # (if applicable)

Closes #31711

### Reason for this change

When we set mapPublicIpOnLaunch=false for public subnets, NAT instances does not get public IP addresses assigned, resulting in non-working NAT instances.

Disabling mapPublicIpOnLaunch is recommended as AWS Config rule (and cdk-nag as well.)
https://docs.aws.amazon.com/config/latest/developerguide/subnet-auto-assign-public-ip-disabled.html

### Description of changes

Support `associatePublicIpAddress`

### Description of how you validated changes

New unit and integ tests.

### Checklist
- [ ] 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*