Add per-object archival-category (classificatie) write endpoint

[rubenvdlinde]

From DocuDesk archiefwet-retention-engine. OR has no per-object archival-category write endpoint at HEAD (only extendArchiefactiedatum). DocuDesk's retention engine needs to set/override the selectielijst classificatie per object. Decision was to build this in OpenRegister. Please add a guarded per-object classificatie write endpoint. Ref: docudesk archiefwet-retention-engine.