ec2-alpha: Allow creation of IPv6-only subnets

### Describe the feature

When creating a new `SubnetV2`, the developer can choose to create an IPv6-only ("native") subnet.

### Use Case

I don't know how to currently do this on CDK level besides dropping down to L1 constructs. Both V1 and V2 of the VPC/Subnet CDK L2 constructs force me to provide IPv4 blocks to subnets. (E.g. [ec2-alpha SubnetV2 ipv4CidrBlock parameter is required](https://docs.aws.amazon.com/cdk/api/v2/docs/@aws-cdk_aws-ec2-alpha.SubnetV2.html#ipv4cidrblockspan-classapi-icon-api-icon-experimental-titlethis-api-element-is-experimental-it-may-change-without-noticespan))

### Proposed Solution

The aim is to generate CloudFormation that looks something like [this example](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html#aws-resource-ec2-subnet--examples--Subnet_with_an_IPv6_CIDR):

```yaml
mySubnet:
Type: AWS::EC2::Subnet
Properties:
VpcId: !Ref myVPC
Ipv6Native: true
Ipv6CidrBlock: !Select [ 0, !Cidr [ !Select [ 0, !GetAtt myVpc.Ipv6CidrBlocks], 1, 64 ]]
AssignIpv6AddressOnCreation: true
```

### Other Information

Possibly part of https://github.com/aws/aws-cdk/issues/30762

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

aws-cdk-lib@2.199.0

### AWS CDK CLI version

2.1016.1 (build 6de56b2)

### Environment details (OS name and version, etc.)

Generic

Start by reading the ec2-alpha SubnetV2 entry point and the AWS::EC2::Subnet example linked in the issue, comparing the required ipv4CidrBlock with the Ipv6Native and Ipv6CidrBlock properties. Done means SubnetV2 can represent an IPv6-only subnet and synthesize the corresponding CloudFormation properties without requiring an IPv4 block.

Written by the indexing model from the issue text.