I'm proposing to move this plugin to subpackage to mitigate the dependency cycle that I stumbled into in D13904. Only one SPA plugin in pipewire package depends on ffmpeg, and this plugin isn't necessary for general PipeWire operations, so it can be safely moved to another package and dependency cycle sdl2->pipewire->ffmpeg->sdl2 can be avoided.

Rebuilt sdl2 with static dependency on pipewire and checked that sdl2, ffmpeg, pipewire and pipewire-spa-ffmpeg packages are co-installable and there is no cycle dependency between them.