Use consolidated storage location for custom covers

[curche]

Describe your suggested feature

Since the app is now migrating to better accommodate scoped storage restrictions (#9591), an alternative to backing up custom covers from /Android/data/, which is mostly inaccessible since Android 11, might be now possible:

current suggestion of this request:

• use ${Tachiyomi Directory}/covers (alongside backup, downloads, local) to store and retrieve custom covers
• rename covers to be more user readable (could also be stored per source per manga as done with downloads, however keeping it in downloads might lead to accidental deletion)

Other details

Possible problems:

• similar to orphaned downloads, we'll now end up with orphaned custom covers (but given that )
• title edit logic might need to also consider custom cover location (previously tracked by https://github.com/tachiyomiorg/tachiyomi/issues/2040, was closed in favor of 1.x suggestions, which was archived recently albeit still an open issue that can be found at https://github.com/tachiyomiorg/tachiyomi-1.x/issues/10)

For Reference:

• #9591
• #5836
• https://github.com/tachiyomiorg/tachiyomi/issues/5059

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 an issue with an extension, I should be opening an issue in the extensions repository.
• [X] I have updated the app to version 0.14.7.
• [X] I will fill out all of the requested information in this form.