ENH: Add functions for File entities

For https://gitlab.indiscale.com/caosdb/customers/cfaed/management/-/issues/29. This introduce a specific treatment of file entities and some other helper functions. This MR also introduces the retrieval with multiple ids.

Test together with f-inline-references branch of complexity wiki (use server profile in local-inline-references). npm pack this repo, then npm install it in the complexitywiki and start the react app. Verify that the functionality introduced here (files, Property.isList, and multi-ID retrieve) work as expected.