feat: Add EpubFileLoader for EPUB file processing

[danik-tro]

Add EpubFileLoader for EPUB file processing.

Solves #160

Changes

• Implemented a new loader type, EpubFileLoader, in rig-core/src/loaders/epub.rs under the epub feature.
• Added an optional dependency on epub-rs for handling EPUB files.
• Extracts chapters from the EPUB file. Currently, the text is retrieved in XML format, as EPUB files are archives of XML files.
• Potential enhancement: Add methods to EpubFileLoader for stripping XML tags to produce plain text.

[0xMochan]

Great work on this, will be reviewing this soon! I love how you added a by_chapter method which matches by_page. Let me know how creating a custom Loader was, I think there are ways to make it cleaner with loader traits but the way the types are setup might seem a bit confusing at first!

[danik-tro]

Great work on this, will be reviewing this soon! I love how you added a by_chapter method which matches by_page. Let me know how creating a custom Loader was,

Thank you for the feedback! I spent some time understanding how the loader works, and that took the most effort. Compared to researching the existing ones, implementing the new loader took significantly less time. I really like the concept of the type state pattern.

I think there are ways to make it cleaner with loader traits but the way the types are setup might seem a bit confusing at first!

Yes, it does look confusing, but it’s effective at the same time. I’m experimenting with a few improvement options, and if something works out, I’ll share an example.