AWS::EC2 - Add AWS::EC2::SubnetCIDRReservation

### Name of the resource

Other

### Resource name

AWS::EC2

### Description

At the moment there is no way to create CIDR reservations in subnets using CloudFormation so I have to do this manually afterwards. It would be ace if this functionality was added so that I can eliminate the manual step in my process.

### Other Details

This functionality currently exists in the API, CLI, and SDK - https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_CreateSubnetCidrReservation.html

Start with the referenced CreateSubnetCidrReservation API documentation and review how this roadmap project records AWS resource coverage. Define the CloudFormation support needed for AWS::EC2::SubnetCIDRReservation and verify that CIDR reservations can be created without the manual post-deployment step; no repository files or tests are identified in the issue.

Written by the indexing model from the issue text.