[downloader] Allow streaming unmerged formats to stdout using ffmpeg

[downloader] Allow streaming unmerged formats to stdout using ffmpeg

For this to work:
1. The downloader must be ffmpeg
2. The selected formats must have the same protocol
3. The formats must be downloadable by ffmpeg to stdout

Partial solution for: https://github.com/ytdl-org/youtube-dl/issues/28146, https://github.com/ytdl-org/youtube-dl/issues/27265