TV shows: automatically add season fanart to series' extrafanarts

[mihawk90]

Is your feature request related to a problem? Please describe.
Not really a problem, more of a quality of life change.

Describe the solution you'd like
Currently extrafanarts need to be selected manually on a series level (this currently also doesn't work but that's a different discussion). If a user already selects fanarts on a season level, it would be nice if those were to (optionally) show up as series extrafanarts as well. It could be a simple toggle in the settings menu. The files can either be copied over, or use hard- or soft-links on supported filesystems (which should be all) instead.
I'm not sure whether Kodi, Jellyfin, or Plex require sequential numbering for extra fanarts, but if they don't then the season fanarts could use a prefix in the filename (e.g. fanart9901.jpg for season 1 and so on) as to not interfere with selected fanarts on a series level (this would also be a way to exclude those files from overriding in case the hard-/soft-link solution is used).

Describe alternatives you've considered
Files can be copies/linked manually, so as noted above it's just a QoL feature.