Add bindings for libgstaudio
There are multiple steps here:
- Add a Gir_Gst*.toml and autogenerate all relevant types, see https://github.com/gtk-rs/gir/
- Make sure the autogenerated code is correct, add manual code as needed
- Write manual bindings for anything that can't be automatically generated (macros, miniobjects, GstAudioInfo/Frame, ...)
- Add src/lib.rs for re-exporting, Cargo.toml, etc