crfs: make it a plugin for fuse-overlayfs

it requires this change in fuse-overlays:

containers/fuse-overlayfs#79

the crfs plugin can be loaded using:

-oplugins=crfs.so,lowerdir=//crfs/DATA/lower

where DATA is the base64 representation of a resource as:

file:///path/to/layer.stargz

or:

https://www.example.com/layer.stargz

Signed-off-by: Giuseppe Scrivano <[email protected]>