feat: Add 'targetCategory' to annotation object

...n/src/main/java/unipotsdam/gf/interfaces/IAnnotation.java

...f/modules/annotation/controller/AnnotationController.java

...va/unipotsdam/gf/modules/annotation/model/Annotation.java

...otsdam/gf/modules/annotation/model/AnnotationMessage.java

...am/gf/modules/annotation/model/AnnotationPostRequest.java

...potsdam/gf/modules/annotation/view/AnnotationService.java

gemeinsamforschen/src/scripts/dbschema/fltrail.sql

...rc/test/java/unipotsdam/gf/interfaces/AnnotationTest.java