API proposal: access WirePlumber or PipeWire configuration files from Lua API

Required by: !412 (merged), pipewire#2210

My proposal is to allow sandboxed scripts to have readonly file access (list directories and read files) in dedicated config folders corresponding to WirePlumber configs: global and user config files.

This allows for implementing new config types like pipewire#2210. This approach would be more performant rather than searching by ObjectManager Interests, so being more scalable long-term.

An alternative is to keep Lua table-based configuration discovery, which sacrifices some performance and usability at scale.