Refactor addAnnotation method for AnnotationTarget=Field

[EgorkaKulikov]

Description

Currently the discussed method looks as follows:

private fun addAnnotation(annotation: CgAnnotation) {
        when (annotation.target) {
            AnnotationTarget.Method -> collectedMethodAnnotations.add(annotation)
            AnnotationTarget.Class -> currentTestClassContext.collectedTestClassAnnotations.add(annotation)
            // TODO: possibly introduce some scope cache like in methods and constructions processing.
            // It may allow to avoid return type in CgAnnotation method.
            AnnotationTarget.Field -> { }
        }

        importIfNeeded(annotation.classId)
    }

The mentioned TODO need to be fixed.
After that, it will be possible to make addAnnotation method return void that is better from the point of view of design.