[FEATURE] Distiguish File loading from Document loading

[JJK801]

Describe the feature you'd like

ATM, File loading (Direct Upload, S3 Source, ...) is deeply correlated to Document loading (Langchain loaders, Unstructured API, ...), but i think it should be separated concerns in order to be able to reuse the Document loading logic.

A good way would be to introducing FileLoader entity that can be an input for DocumentLoader entity.

Additional context

Example: I am unable to load files from S3 without using Unstructured API for chunking (even if it is possible to use any loader at code level)

Solution: I should have a S3File (FileLoader) node that can be linked as an input for UnstructuredFile (DocumentLoader) node or any other document loader accepting files.