Generates data fetcher interface for query types

[zaenk]

The Java generator has an option to generate example data fetchers that can be copied into the codebase and extended as needed, but the kotlin generator has no such option.

This PR implements example data fetcher generator for kotlin with a twist: instead of generating a a class with TODO bodies, it generates an interface, for now.

This is the first in a series of PRs that would add an option similar to OpenAPI kotlin-spring generator interfaceOnly option.

Interfaces generated by this way would only be usable if the example output directory is set to the generated code directory. However, the generated interfaces would not be useable until Netflix/dgs-framework#320 is resolved.

The final solution would enable the development of schemas in a separate repository. Automation would use this generator to generate data classes for types and interfaces for data fetchers and mutations that server component must implement. The generated sources would be packaged and published as a jar. in the final solution, this interface generation behavior would get it's own configuration option/flag.

The current PR introduce just enough change that would enable me to further experiment with code generation in small iterations without diverging much from the mainline.

edit: elaborates scope, links blocker PR in dsg-framework

[ilterpehlivan]

Hi,
This PR would be answering my question here ? Netflix/dgs-framework#308 (comment)

Thanks

[zaenk]

@ilterpehlivan yes, essentially it does, original discussion: Netflix/dgs-framework#234

[zaenk]

rebased, tests are now passing