Adding option to create @Generated annotation on generated code

[yonatang]

Following the request in #420 and discussion in Netflix/dgs-framework#1147, this PR provides an option to generate @Generated annotation over the generated code (both for Java and Kotlin code generators).

The code will detect which @Generated annotation should be used (javax.annotation.Generated for JVM <= 8 or javax.annotation.processing.Generated for JVM >= 9) and will add that for every relevant TypeSpec (classes, data classes, objects, interfaces, enums, etc). The annotation will contain the generation timestamp, and the code generator name will be com.netflix.graphql.dgs.codegen.CodeGen, as the @Generated spec requires

The Java and Kotlin unit tests will use a schema to generate all relevant types, and will verify they: a) have the @Generated annotation; and b) can get compiled properly with the annotation present.

[srinivasankavitha]

Thanks for the PR contributions!

[yonatang]

I'll change to the suggested name, and I can see the linter is angry with me. I'll fix both.

[bartebor]

@yonatang, thanks for picking up my issue :)

As I see a @javax.annotation.processing.Generated annotation is added to source files and this is great.
Unfortunately above anotation has retention SOURCE and thus is dropped by the compiler so coverage checks don't see the annotation.

Could you use some other annotation (@lombok.Generated) or create custom annotation (no external dependencies) with retention RUNTIME or CLASS, so it would be retained in compiled classes?

The problem with java-provided annotation(s) is explained here: jacoco/jacoco#831 (comment)

[yonatang]

Thanks @bartebor, I was surprised to discover the SOURCE retention annotations are not picked by Jacoco. I guess the best approach is to add a custom CLASS retention @Generated annotation, which Jacoco seems to like better.