Support AWS PrivateLink / VPC Endpoints for Hosting

### Before opening, please confirm:

- [X] I have checked to see if my question is addressed in the [FAQ](https://github.com/aws-amplify/amplify-hosting/blob/master/FAQ.md).
- [X] I have [searched for duplicate or closed issues](https://github.com/aws-amplify/amplify-hosting/issues?q=is%3Aissue+).
- [X] I have read the guide for [submitting questions](https://github.com/aws-amplify/amplify-hosting/blob/master/CONTRIBUTING.md).
- [X] I have removed any sensitive information from my code snippets and submission.

### App Id

NEW_APP

### AWS Region

eu-central-1

### Amplify Hosting feature

Not Applicable

### Is your feature request related to a problem? Please describe:

Our internal customers are using desktop computers without internet access (neither via proxy, nor directly). We cannot use AWS Amplify to host websites of any kind for these internal customers, since we cannot use the "usual" pattern of Customer->PrivateLink->NLB (with static IP)->VPC Endpoint.

This is somewhat similar to #794, but that issue doesn't really spell out the customer-facing use case of this. #36 is also related, but again, a different use case. In our use case, the users **cannot** access the amplify public endpoint at all and have to use an internal IP connected via PrivateLink.

### Describe how you'd like this feature to work

I'd like there to be a VPC Endpoint type for AWS Amplify (similar to how there is one for AWS API Gateway), which allows me to access hosted sites using an ENI IP, which I can point a Network Loadbalancer to.

No implementation files or tests are identified in the issue. Start by reviewing related issues #794 and #36, then the linked FAQ.md and CONTRIBUTING.md; define the acceptance criteria for accessing hosted sites through an Amplify VPC Endpoint, ENI IPs, and PrivateLink.

Written by the indexing model from the issue text.