Create binding for pw_core_sync() as pipewire::Core.sync()

This Merge Requests adds a binding for pw_core_sync() by adding a sync() method to pipewire::Core.

I've also added an examples/ folder containing an example that shows how to use that method, analogous to the existing upstream C version. It's not quite complete as we don't have a method for adding listeners to the registry yet, but perhaps this can be merged already, too. The example can be run with cargo run --example roundtrip.