Recommended upgrade path from old multi-API stack

[crrobinson14]

We are upgrading a v1 stack with two APIs defined in a single stack to the latest version of the plugin. As noted in https://github.com/sid88in/serverless-appsync-plugin/blob/master/doc/upgrading-from-v1.md multi-API stacks are no longer supported, so we separated the environment into two stacks. So far so good.

However, we have a legacy app that would be very difficult to update in terms of the API URL it's wired to. Breaking out the stacks like this changes the URLs (we've confirmed this in dev, not that it was unexpected). There is a suggestion here: You can probably use [custom domains](https://github.com/sid88in/serverless-appsync-plugin/blob/master/doc/custom-domain.md) to workaround that, if that's an option. We will be testing this to confirm but I also wanted to post here to see if you had any new thoughts on possible options here and/or if you still felt this was the correct approach. The Custom Domains method seems to allow us to set our OWN domain, but not necessarily take over one of the randomized URLs auto-generated by AWS in deployments that don't have them. Perhaps there is some way to set this to refer to an existing URL?