(vpc): Access the Internet Gateway for a VPC loaded via "ec2.Vpc.fromLookup"

### Description

Add the ability to look up the Internet Gateway for a VPC loaded via `Vpc.fromLookup`.

### Use Case

I'd like to be able to add a subnet route like this:

```javascript
const vpc = ec2.Vpc.fromLookup(this, 'vpc', { 'vpc-XXXXXXXX' });

const subnet = ...;

subnet.addRoute('IgwRoute', {
routerType: ec2.RouterType.GATEWAY,
routerId: vpc.internetGatewayId,
destinationCidrBlock: "0.0.0.0/0",
})
```

but `vpc.internetGatewayId` comes back as `undefined` for me. Is this expected?

### Proposed Solution

I believe there is some relevant context here:
https://github.com/aws/aws-cdk/issues/5327#issuecomment-961894054

### Other information

_No response_

### Acknowledge

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

Start by tracing the Vpc.fromLookup entry point and the internetGatewayId property; the issue's example shows the lookup and route use case. Review the linked issue comment for context, and consider the work done when a looked-up VPC exposes its Internet Gateway ID for the shown route configuration.

Written by the indexing model from the issue text.