Reading raw EPUB item data in v3

[smoores-dev]

Hello! It's me again, working on the Storyteller iOS app this time.

With v2 (and in v3 on Android), I used an onCreatePublication callback in Publication (now PublicationOpener) to add mediaOverlay properties to all of the links in the readingOrder. Basically, I parsed the OPF manually, and then looked for any media-overlay properties and added those to their respective links.

In the Swift toolkit, I'm not immediately seeing how to do this. Running into two issues:

1. container.readData seems to be internal, and I'm not sure whether there's another way to read the raw file data from the container?
2. Even if container.readData was public, it's now async, but onCreatePublication needs to be sync.

Hoping that there's a way around this that doesn't entail forking... like all of PublicationOpener :/ Any ideas?

Here's the current implementation with Readium v2, in case that's helpful! https://gitlab.com/storyteller-platform/storyteller/-/blob/main/mobile/modules/readium/ios/BookService.swift#L25-67