feat(elasticloadbalancingv2): subnet mappings for network load balancer (#33736)

### Issue # (if applicable)

Closes #9696.

### Reason for this change

Cloudformation supports the configuration of subnet information through `subnetMapping` more detailed than using `subnetIds` but AWS CDK doesn't support this.

https://docs.aws.amazon.com/ja_jp/AWSCloudFormation/latest/UserGuide/aws-properties-elasticloadbalancingv2-loadbalancer-subnetmapping.html

### Description of changes

- Add `subnetMappings` to `NetworkLoadBalancerProps`
- Define `SourceNatIpv6Prefix` class and `SubnetMapping` interface at `base-loadbalancer.ts`

### Describe any new or updated permissions being added

None

### 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*

Author: badmintoncryer