composefs deploy: Store cfs signature in .ostree.cfs.sig file

composefs deploy: Store cfs signature in .ostree.cfs.sig file

In many cases, such as when using osbuild, we are not preparing the final
deployment but rather a rootfs tree that will eventually be copied to the
final location. In that case we don't want to apply the signature directly
but when the deployment is copied in place.

To make this situateion workable we also write the signature to a file
next to the composefs image file. Then whatever mechanism that does
the final copy can apply the signature.