Elixir libraries for external services.

The Library module exists because there aren't Elixir libraries for all of the APIs out in the world, and sometimes we have to implement our own. It's designed as a separate app because that makes it easier to ensure that it's isolated + idempotent, and can be extracted into a separate repo entirely if necessary.

Similarly we have a Utility app for related functions that we want to use across every app in the umbrella, but which don't relate to the business logic directly.