AWS::EC2::TransitGatewayRoute - Support Import Operation

### Name of the resource

AWS::EC2::TransitGatewayRoute

### Resource name

_No response_

### Description

Currently it is possible to import a Transit Gateway route table but not its routes. This is blocking us from improving our Transit Gateway disaster recovery procedures by defining our existing core network as code.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-transitgatewayroute.html
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/resource-import-supported-resources.html?icmpid=docs_cfn_console

### Other Details

_No response_

Start with the two linked AWS CloudFormation documentation pages to confirm the import requirements for AWS::EC2::TransitGatewayRoute. No repository file, test, or entry point is named, so locate the corresponding coverage entry and validation path before proceeding. Done means existing Transit Gateway routes can be imported, like their route table.

Written by the indexing model from the issue text.