Add initial libgstsdp, libgstrtsp and libgstrtspserver bindings

Only automatic bindings for now, which is enough to allow implementing a simple rtsp-server example.

Depends on https://github.com/sdroege/gstreamer-sys/pull/8

Uses a new girr feature proposed at https://github.com/gtk-rs/gir/pull/539 to make doc regeneration easier.