Extracting EPUB encryption into a DRM extension

[mickael-menu]

Currently, the encryption JSON schema is part of the EPUB extension.

Accordingly, the type is named with the EPUB prefix in the Swift project.

But now with LCPDF and soon audiobooks, this model is not limited to EPUB anymore, so I suggest extracting it to a new DRM extension.

[HadrienGardeur]

Now that we've divided extensions into modules and profiles, we could definitely move the encrypted property to its own module.

[llemeurfr]

I totally agree. I'll do it when I take a look at what Hadrien did last week.