Validation and filter feature

There have been a number of issues reported (linked in #882) where it seems that user's data files have been corrupted. Some of these things should be fixed on the frontend, so that it makes sure invalid data is not sent. However, the backend should also check, e.g. by doing a RDF validation as suggested in #882, but we could also imagine SHACL validation, etc. If Inrupt becomes a large POD provider, it may also come with legal requirements.

Also, validation may not only be a boolean accept or error, but possibly also filtering to accept valid parts. For v.next, we need to have an architectural element that does this, but we may need to address parts of this problem already for 5.0.0.

Consider this a super-issue for discussing what should be in 5.0.0, and if we should try to make that reusable in v.next. Also, we may discuss if we do not attempt to solve it on the backend and refer to frontends to do it.

Beginne mit dem Lesen der in #882 verlinkten Berichte und der Diskussion über RDF- und SHACL-Validierung. Kläre, ob die Validierung im Frontend, im Backend oder in beiden erfolgen soll, und definiere den Umfang für 5.0.0 gegenüber v.next. Als erledigt gilt die Vereinbarung einer wiederverwendbaren Architektur und klarer Anforderungen für das Zurückweisen oder Filtern ungültiger Daten.