add transit_gateway_route module

[paulczar]

SUMMARY

New module for creating transit gateway routes

ISSUE TYPE

• New Module Pull Request

COMPONENT NAME

transit_gateway_route

ADDITIONAL INFORMATION

This allows you to create transit gateway routes, without them transit gateways are pretty useless.

I haven't created tests for it, and don't really have the time, I'm hoping someone else is interested enough in the feature to run with that.