Improve internal data consistency and typing

Different softwares format ActivityPub objects differently, because the specs are quite loose. For example an object might have attachments that either need to be fetched separately or are resolved already.
Currently, our handling for less common formats is inconsistent and the json needs to be checked in all locations (instead of validating in once when receiving data).

By developing a mechanism to specify the exactly desired internal target format and providing type safety, we can improve code stability and make future app development easier. To aid in code-reusability, this will be developed in the SemApps repository.