Implement helper functions for generating osbuild stages for a slices of
`fsnode` types, such as Files and Directories. The generated stages will
ensure that the provided FS nodes will be created in the FS tree and
will have their respective properties set (such as ownership, mode,
etc).
These functions are not yet used by any pipeline code, but the idea is
that they will be used in pipeline generator functions to create custom
directories and files based on the pipeline-specific customizations.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
