Auto-generate bindings from gir files

[neilcsmith-net]

Longer term we have been recommended by GStreamer developers to (re)look at auto-generation of bindings, but based on our existing GLib / GObject support. The initial stage of this will be to look at auto-generating the lowlevel package in a fully compatible way so that the API outside of that package remains stable. Auto-generating the API level packages could happen later.

This is similar to how the Rust bindings are structured and built, which will be one project used as a guide in this process. These are similarly split in a lowlevel (https://github.com/sdroege/gstreamer-sys) and high-level (https://github.com/sdroege/gstreamer-rs) way.