⚠ Support multiple hosted zones within a single VPC

[lorenyu]

Currently, the network configuration only contains one domain_config attribute which means there can be only one subdomain in the VPC. This is limiting for projects with a VPC that maps to multiple environments. For example, you might want a VPC for all lower environments, and you might want a separate subdomain for each lower environment. You could just have a single lowers.foo.com subdomain, then put everything under that, but it seems better to be able to hide the VPC from the URL.

This would necessarily be a breaking change since we're changing the network config object structure, so we want to make sure to include migration notes as part of the PR.