filesystems: Add fuse-gstfs-ng. Replaces fuse-gstfs.

gstfs-ng is a FUSE-based file system that works as a dynamic
gstreamer-based transcoder over an existing directory.

For example, this can make your FLAC music archive appear as an
MP3 or OGG archive.

Transcodings can be cached in memory and persisted to disk.

(nia)