BREAKING CHANGE: generate marker interfaces for union types by default

[danadajian]

📝 Description

• GraphQL Kotlin allows for 2 ways of representing union types in Kotlin: marker interfaces and annotation classes. Marker interfaces (i.e. empty Kotlin interfaces) should be the default way we generate Kotlin code from GraphQL union types. This way, we enforce that resolvers implement one of a few classes that inherit from the interface, rather than enforcing the use of Any!
• This is a breaking change which will make the default behavior to generate marker interfaces from union types, but leave the option to generate annotation classes (via unionGeneration: ANNOTATION_CLASS).
• This change also injects some defaults into the parsed config object and refactors a bunch of code for repo maintainability.