On demand pipewire modules loader script

This allows loading pipewire modules on demand by setting metadata. It is useful to dynamically load loopbacks or network modules without having to start a new pipewire process with a hardcoded config file.

The key name should start with "wireplumber.on-demand-module." and all the characters after that are considered a unique id for this instance of the loaded module.

The value should be a json object in the same format as a module entry in pipewire.conf, containing "name" and "args"