auto download podcasts in app

[brownphotographic]

Background
As a user I would like the app to be able to auto download podcasts episodes and books. This is particularly helpful for a self hosted solution so the user doesn't have to constantly switch servers (and launching tailscale) when leaving the house. Also useful when traveling on a plane... Having a backlog of podcasts already downloaded is super helpful so the user doesn't have to manually download episodes before a flight.

Requirements

1. User settings in app to set download frequency for checking for new podcasts and books (e.g. daily at specific time, hourly, weekly).
2. User settings in app to set number of episodes or books to download (global setting across all podcasts is simplest).
3. User settings in app to allow background sync option
4. User settings in app to download over WiFi only (default) and cellular data. May need to implement a timeout if no connection is established.
5. User setting to toggle auto download on or off for books and podcasts
6. When settings applied, podcasts or books auto downloaded

[nichwall]

This belongs in the app repository. Duplicate of advplyr/audiobookshelf-app#613