Allow annotation processors for tests

[aggmegalou]

Add opt-in support for annotation processor dependencies (e.g. Lombok) to be available for test compilation when using Gradle. When metadata sets annotationProcessorForTests: true, the dependency is added to both annotationProcessor and testAnnotationProcessor configurations.

• Add annotationProcessorForTests flag to Dependency metadata
• Add TEST_ANNOTATION_PROCESSOR scope to DependencyScope
• Map scope in GradleBuildWriter and MavenBuildWriter
• Add testCompileOnly.extendsFrom(testAnnotationProcessor) in Gradle customizer
• Resolve test annotation processor dependency in DefaultProjectRequestToDescriptionConverter
• Add MetadataBuildItemMapper.toDependency overload with scope override
• Add tests and BuildComplianceTests for end-to-end verification

Closes gh-1380

[mhalbritter]

Hey, thanks for the PR. After looking at it and the current arrangement, I think we have more refactoring to do, which is out of scope for this PR.

• I don't like the configuration inheritance between annotation processors and compile scope for Gradle. For dependencies which are both an annotation processor and a library, it'd be better to add them as a annotation processor and as a "normal" dependency. I've already added testCompileOnly support in Add support for Gradle's testCompileOnly dependencies #1777. This also affects Annotation processors are added to compileOnly and annotationProcessor but we configure compileOnly to extend annotationProcessor #1379.
• Annotation processors are only applied to main sources in Gradle, but on Maven they are applied to both main and test sources
• Test annotation processor support is missing right now. If we add it, I think we should change the behavior: If an annotation processor is added, it's configured as annotationProcessor in Gradle and as an annotation processor ONLY for the main sources in maven. If a test annotation processor is added, it's configured as a testAnnotationProcessor and ONLY for the test sources in Maven.

I'll open separate issues for this and close the PR. Thanks anyway!