Handle unions, intersections and lists when generating sample GQL

[cliftonmcintosh]

This corrects some problems with generation of sample GQL by handling the following return types:

1. Unions
2. Interfaces
3. Lists
4. Non-null lists

Testing

• Run mix test and make sure unit tests pass
• Run mix test and verify the output in the markup files
• Run this version of the code on a set of sample repos and verify that it succeeds and the output looks correct.

I ran this on 9 repos in my organization and verified that it succeeded and the output looked correct.

Sample output

For each of these, see the updates to schema.json for more specifics.

Unions

In this example, the return type is LoginResponseV2, a union of LoginResponse and UnexpectedRequest.

Interfaces

In this example, the return type is Character, which is an interface implemented by Human and Droid.

Lists

In this example, the return type is a list of Droid objects.

Non-null return types

In this example, the return type is a non-null list of Human objects.