This PR does two things.

1. Adds a helper method `then()` that makes it easier to chain assertion
   api calls together. Yes, it is possible to grab the underlying `node`
   and call `addDependency`, but I think `then` is a more intuitive
   experience.

Look at `integ.log-group.ts` to see where I updated a test from
`addDependency` -> `then`

2. Added an `ApiCallBase` class and renamed the api call interface. This
   will make it easier to add more types of Api Calls in the future
   (`HttpApiCall` coming soon*)