Define public API surface

[vlaci]

Unblob is converted to a py.typed library in #352. After that change we are also able to define library-private and public modules and types. We should expose the functionality needed for external handlers and for running unblob as library and make the related code public.

https://github.com/microsoft/pyright/blob/main/docs/typed-libraries.md#library-interface

[kissgyorgy]

It is better and easier to start with the absolute minimal public surface and open up later, because once we make something public, we essentially can't "take it back", because people can start relying on those. Also it's easier to refactor internal implementation when we don't expose too much.

Therefore I suggest to expose only the main entry point process_files and the related objects (ExtractionConfig and Report), and later we can open other functions as necessary.