Model a Direct Connect Transit Gateway so it can be included in TGW routesTo

[apmclean]

Is your feature request related to a problem? Please describe.
For use-cases where the Transit Gateway is associated with a Direct Connect Transit Gateway a model should exist that permits including the DX Gateway attachment as a destination for a routesTo: in supported Routes.

Describe the solution you'd like
A model for a DX Gateway in the configuration file. The required configuration items would be imported as existing (route table and attachment identifiers) similar to how an existing TGW or existing VPN is imported today.

CloudFormation currently has no support (aws-cloudformation/cloudformation-coverage-roadmap#876) for creating the Direct Connect Gateway itself, but we can model it by importing required attributes to support routesTo: within the Transit Gateway Route tables.

When a routesTo: destination is the modeled Direct Connect Gateway - static routes, dynamic routes and/or default routes would be configured in the same way they are today for VPC and VPN connections.