AWS::EC2::VPC supports referencing Route Table

## 1. Title
#### AWS::EC2::VPC-RouteTable

## 2. Scope of request
AWS::EC2::VPC creates a default/main route table, but it is not possible to change the route table or edit the routs in the same template.
By adding RouteTable to the Get::Att, customers would be able to get a reference to the main route table and add routes to it.

## 3. Expected behavior

#### In Create,
Get::Att RouteTable on the VPC should give you the ID of the main route table.
#### in Update,
same as Create

## 5. Helpful Links to speed up research and evaluation

The premium support article for changing the main routes points to a custom resource that returns the main Route Table for the VPC. That should be native for the VPC resource: https://aws.amazon.com/premiumsupport/knowledge-center/cloudformation-route-table-vpc/

## 6. Category (required) - 3. Networking & Content

Start with the linked AWS Premium Support article and the AWS::EC2::VPC resource behavior described in the issue. Determine how Get::Att should expose the main route table during Create and Update; done means the VPC returns its main route table ID and routes can be edited in the same template.

Written by the indexing model from the issue text.