Implement path serialization

[JAB1305]

Previously the $path serialization modifier was not supported.

This introduces a central helper to collect / generate idShortPath lists, allowing either deep or only direct children (following level query param). It also introduces support for the endpoints reported in #643.

Fixes #643.

[hpoeche]

Good job, the implementation seems correct to me. I'd like to throw in two ideas, that would generalize this a bit:

1. We could wrap this implementation of server/app/adapter/path_serialization.py into a subclass of the Python json.JSONEncoder. This would remove depending on the ResultToJsonEncoder to encode the created list[str] structure correctly.
   Additionally, it allows us to handle serialization of a list of objects directly in this class, instead of aggregating lists afterwards (e.g. for get_submodel_submodel_elements_path()).
   We can move down the choice of the encoder into the JsonResponse class, similar to StrippedResultToJsonEncoder, and thereby separate data fetching from encoding.
   Finally, this allows us to drop the _assert_path_supported() and assert_json_response() methods.
2. This newly extracted Encoder class could be moved to the sdk package to allow users to use it. This naturally follows our separation of the spec parts, because the Path-Serialization is defined in Part 1: Metamodel, which is implemented in the sdk package.

Feel free to reach out, if I left anything unclear or you would like to discuss this further.