feat: loader for epubs

[0xMochan]

• I have looked for existing issues (including closed) about this

Feature Request

Add a loader type that can work with the EPUB format (ebook format).

Motivation

Oftentimes, PDFs are unreliable format for loading. While it is the most common and likely will be continued to be used, adding more loader types adds flexibilities in people's abilities to construct RAGs.

Proposal

• Create a new loader type (rig-core/src/loaders/epub.rs) EpubFileLoader

Alternatives

• Use a Epub -> PDF converter before using PdfFileLoader

Notes

• If this introduces a new dependency, it should add it as an optional feature (similar to the lopdf lib) and only just include the epub.rs loader if that feature is selected!