Add route table IDs to VPC context provider

[jkenn99]

I would like to create Route resources on an existing VPC.

My use case looks something like this:

const new_vpc = new ec2.VpcNetwork(this, 'new_vpc' {
  // ...
});
const vpcId =  'vpc-xxx';
const existing_vpc = ec2.VpcNetwork.importFromContext(this, 'existing_vpc', {
  vpcId: vpcId,
});
const pcx = new ec2.CfnVPCPeeringConnection(this, 'new_vpc-existing_vpc', {
  vpcId: vpcId,
  peerVpcId: new_vpc.vpcId,
});
import_vpc.publicSubnets.forEach((subnet,index) => {
  new ec2.CfnRoute(this, 'route-' + index, {
    routeTableId: subnet.routeTableId,
    destinationCidrBlock: new_vpc.cidr,
    vpcPeeringConnectionId: pcx.ref,
  });
});

[mhuebner]

Related to #2926 but your code does not match to the structure of current CDK-Version (0.36.0).