Refactor open-next e2e tests to be compatible with Multi Site

Currently, we use example Next.js apps and e2e tests from open-next by using git submodules. This has worked since `cdk-nextjs-standalone` is using a subset of open-next/NextjsSite's features. However, now that we've added multisite support and potentially future features that NextjsSite doesn't support, we're going to need to pull the Next.js example apps and e2e tests into this repository.

To be more explicit, in order to deploy an example multisite, you have to manually change the next.config.ts for open-next/examples to have `basePath`. That's a pain. Additionally, the e2e tests use `page.goto("/");` which is incompatible with multisite.