Support for multiple storages at once (SD & internal) for offline reading/downloaded

### Describe your suggested feature

A feature to have multiple storage locations would be nice for users with lower storage, A good way I can see this being properly implemented is by having a "main" storage and "secondary" storage, for example, the main storage will act the same as how storage currently works (backups, downloads, etc), the "secondary" storage will act as another storage location only for downloaded content, more of a "backup" thing.

### Other details

_No response_

### Acknowledgements

- [x] I have searched the existing issues and this is a new ticket, **NOT** a duplicate or related to another open or closed issue.
- [x] I have written a short but informative title.
- [x] If this is a request regarding an extension, I should be opening an issue in the extension's repository.
- [x] I have updated the app to version **[0.16.4.3](https://github.com/aniyomiorg/aniyomi/releases/latest)**.
- [x] I will fill out all of the requested information in this form.