Support for creating a transit gateway attachment to a VPN via CFN

### Name of the resource

AWS::EC2::VPNConnection

### Resource name

_No response_

### Description

To be able to attach a VPN to a TransitGateway resource via CloudFormation in such a way that IP addresses which are assigned to VPN tunnels won't change.

### Other Details

To achieve this via cfn : https://docs.aws.amazon.com/vpc/latest/tgw/tgw-vpn-attachments.html

Start with the AWS Transit Gateway VPN attachment documentation linked in the issue and the AWS::EC2::VPNConnection resource behavior. Determine the CloudFormation properties needed to create the attachment while preserving assigned VPN tunnel IP addresses. Done means the resource supports this configuration and coverage verifies the resulting attachment behavior.

Written by the indexing model from the issue text.