pubsys: Add support for separate environments and waves

**What I'd like:**
It would be great if `Infra.toml` could add support for "waves" of AWS regions, making it possible to release to different sets of regions at different times while still keeping everything in a single config file.

Similarly, it would be great if waves lived under "environments", so that different accounts/regions could be allocated for different purposes within a single document.

**Any alternatives you've considered:**
The problem can be solved now with multiple config files for different use-cases, but this can become cumbersome to manage.

Start by locating the pubsys entry points and the Infra.toml configuration handling. Define how environments and waves should be represented for separate AWS accounts and region groups, then verify that one configuration can support releases to different waves at different times. No tests or specific files are named in the issue, so the relevant implementation and test locations must first be identified.

Written by the indexing model from the issue text.