Feature Request: Custom domain names for Private AppSync APIs

## Description

Custom domain names are currently supported for **public** AWS AppSync APIs (resolving [#5](https://github.com/aws/aws-appsync-community/issues/5)). However, **private** AppSync APIs (which can only be accessed from a VPC) do not support custom domain names.

## Proposed Solution

Allow associating a custom domain name with a private AWS AppSync API, as is possible for Private APIs in AWS Apigateway .

## Related

- #5 Original feature request for custom domains (for public APIs)

Start by reading related issue #5 and the AWS AppSync private API behavior described here; no repository files or tests are named. Determine the requirements for associating a custom domain with a private API, with completion meaning private AppSync APIs support custom domains while remaining accessible only from a VPC.

Written by the indexing model from the issue text.