rofl-containers: Expose app metadata in the filesystem

[kostko]

ROFL apps can have associated metadata (string key-value pairs) which is currently not exposed inside the ROFL app itself. In containerized apps using rofl-containers we can expose it in the filesystem (e.g. each item would live in something like /run/rofl/metadata/<item>) so that individual items or the entire thing can be easily mounted into containers via volumes.