Allow VPCs to have a default gateway via VPN gateway

[MentalPower]

Use Case

You can currently create a VPC with isolated subnets and a VPN Gateway via something like:

const vpc = new ec2.Vpc(this, 'Vpc' {
  vpnGatewayAsn: 1234,
  vpnGateway: true,
  subnetConfiguration: [{
    subnetType: ec2.SubnetType.ISOLATED,
    name: "",
  }],
}); 

However, there's currently no way for those isolated subnets to treat the VPN gateway as their default route. This case is common in setups where all internet-bound traffic goes through another account/VPC.

Proposed Solution

const vpc = new ec2.Vpc(this, 'Vpc' {
  vpnGatewayAsn: 1234,
  vpnGateway: true,
  subnetConfiguration: [{
    subnetType: ec2.SubnetType.ISOLATED,
    name: "",
  }],
  vpnDefaultGateway: [{
    subnetType: ec2.SubnetType.ISOLATED,
  }],
}); 

This configuration will then call the existing subnet.addDefaultInternetRoute() method (currently only used for Internet Gateways) passing in the selected subnets and the newly created VPNGW attachment.

Other

• 👋 I may be able to implement this feature request
• ⚠️ This feature might incur a breaking change

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This is a 🚀 Feature Request

[rix0rrr]

I'm wary of adding more bells and whistles to the existing VPC class.

Treating this as duplicate of #5927