DRAFT: Implement multi-part audiobook support

[Samson50]

Modify the server to handle audiobooks which are split into multiple audio files. This assumes all files for a single audiobook are in their own directory and each directory only contains files for a single audiobook.

By itself, the changes in this merge request allow the server to handle resolving multi-part audiobooks into a single parent AudioBook entity and serve to the client under the normal books view. Playback can be started from the beginning of the audiobook and continue through all chapters but cannot be "resumed" once the play session is stopped.

When combined with the changes in the web pull request, playback can be resumed from the most recent stopping point from the books list view or from the "Continue Listening" entry.

Changes

• Changed AudioBook entity to a subclass of Folder - serving as a parent for the chapters/files of an audiobook
• Added entity for AudioBookFile representing the individual playable media files of an audiobook
• Added conditionals for AudioBook/AudioBookFiles to controllers/managers to enable expected playback behavior

Issues
Fixes #10668

[jellyfin-bot]

This pull request has merge conflicts. Please resolve the conflicts so the PR can be successfully reviewed and merged.