[route53] Allow addVpc for imported PrivateHostedZone

Allow something like this so we can import a private hosted zone not generated from the stack but be able to add VPCs to it.

``` javascript
const hostedZoneId = 'Z43243432432'
PrivateHostedZone.fromPrivateHostedZoneId(this, `R53PrivateZone-${hostedZoneId}`, hostedZoneId).addVpc(vpc);
```

### Use Case

We have a stack that generates our DNS topology. The DNS topology has hosted zones which I would like to subscribe to when I generate a VPC from a stack i own.

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

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This is a :rocket: Feature Request

No file or test is named. Start by locating the Route53 PrivateHostedZone implementation and the fromPrivateHostedZoneId entry point, then inspect how addVpc handles imported zones; done means an imported private hosted zone can add the supplied VPC without requiring the zone to be created by the stack.

Written by the indexing model from the issue text.